To be honest I am not quite sure how Sonic games became less and less appealing. I guess the lack of innovation and increasingly bad-ass style in the newer games made it possible.  
I really liked all the Sonic games on Genesis, even the semi-3D game was pretty nice at the time. Hell I even enjoyed playing Sonic R on my Saturn, it wasn't the best racing game but was still good enough for me to care about it. 
And when the Dreamcast was released Sonic had an outstanding comeback in 3D. I liked the game just as much as Mario's first appearance in 3D... well almost at least. It wasn't perfect but I didn't expect that back in 1999.
Sonic Adventure 2 was a decent sequel but nothing special in my opinion.  
 
And that's about the time those games really started sucking. Sonic Heroes was pretty boring and I won't even start on the current gen game from 2006. And as for the 2D games: to me they mostly feel like remakes :-P 
 
so what's your opinion, am I the only one who thinks that Sonic is a relict?

The only good part of Sonic is the first level of a Sonic game, anything after that is usually terrible. Just my point of view.

Nah man, Adventure sucked too.

We grew up...

What? Sonic Adventure 1 was the awful game that marked the end of the series. Sonic Adventure 2 just drove the nails into the coffin. 
 
Sonic Heroes was a bad game, but at least I can commend it for at least trying to mimic the old games (none of that open world/human bullshit) which at least makes it a better Sonic game than Adventure ever was.

" Nah man, Adventure sucked too. "

Shadow. As soon as Shadow became a mainstay, Sonic games went totally downhill. 

I don't understand why everyone seems to forget the handheld games like Sonic Advance and Sonic Rush. They were pretty good. 
And I think everyone thinks that Sonic is a relic.  

And when the Dreamcast was released Sonic had an outstanding comeback in 3D.    

The last good Sonic game I had played before Sonic 4 was eventually released was Knuckles the Echidna and Chaotix for the 32x. It was basically the original concept for Sonic the Hedgehog 2 re-imagined as another title completely with brand new characters, who all supported Knuckles as the main character. So yeah, technically, it wasn't a Sonic title but it was a Sonic spin-off.
 
The whole chain system in Chaotix was brilliant I thought and for those who don't know - Sonic 2 was supposed to use the same mechanics as this, where Sonic would be attached to Tails by a metal chain but this idea was ditched eventually........however, there is a mod called Sonic and Crackers out there that pulls it off well. 
 
But yeah - I think Sonic really began to fail as soon as Travlers Tales got a hold of it and produced the abysmal Sonic 3D and then from there, it was all downhill for Sonic, as he tried and failed to enter the world of three dimensional gaming.

They kept milking the franchies.  
Just like it is with other milked franchieses 

Could it be the concept of Sonic the Hedgehog is awful? 

@Andrela

" The only good part of Sonic is the first level of a Sonic game, anything after that is usually terrible. Just my point of view. "

I think Adventure was a step into the right direction at least with one foot the other one went bogus (I guess metaphors are not my thing :-D), I just played it last week and still was kind of nice. I also disliked the "human bullshit". In addition Sonic should look cute and funny instead of serious and uber-kuhl. And fuck that Shadow character, he's as unlikeable as it gets.  
   

Hmm I guess it's true that handheld Sonic games weren't that bad, the only really bad one I can think of is Sonic on N-Gage. And that was due to the stupid screen.

I loved Adventure 2 but didn't like the first one. 
 
Sonic Heroes was where it took a nose dive.  It was just boring.  After that they kept pushing new characters with shitty controls and to many glitches.  Sonic Adventure 2 had more characters to play as but Shadow was pretty much Sonic, Tails and Robotnik were the same but they didn't feel like shit when playing them and might I say... fun.  Knuckles and Rouge were annoying but I was able to get through them pretty quick. 
 
The Wii games have over simplified an already simple concept while also making him look slow.
 
I also noticed after 06 there was an abundance of people who proclaimed they never liked Sonic or that every game after Sonic and Knuckles sucks.  06 must have pissed enough people off for them to retroactively hate the Sonic games. 
 
I think the Adventure games haven't aged that great but they can still be pretty fun and they were good games back then.

also I think the portable sonic games are still pretty good (advance 2, maybe not but whatever)

I liked the Adventure game a lot. Other than that Sonic is pretty irrelevant now.

I think the big thing is that Sega wanted to really differentiate Sonic from Mario around the Dreamcast/Nintendo 64 era. Rather than go from 2D to 3D with the same gameplay, which is (debatable) what Mario 64 did. They thought they could add more by putting in voice acting, setting it in the real world, and trying to give it a "deeper" plot than Mario. Which as time has shown just failed.
 
The other big problem is the "Massive Speed" feeling that the 2D games created is just really really hard to convert to 3D without basically taking control away from the player. Since Mario never had to deal with that expectation it easily succeeded at going from 2D to 3D.
 
Also everybody hates new characters that don't add anything interesting.

I'd like to do a study to figure out the age of Sonic fans compared to where the latest game that appealed to them is in the series. It seems like the older you are, the less you liked more recent Sonic games.  
 
For instance, I was a fan of everything from Sonic 1 through Sonic 3 and Knuckles (later discovering that CD was also a pretty good Sonic game, but at the time, I didn't own a Sega CD and never got to check it out). The Giant Bomb crew is a little older than me, and they all seem to agree it ended with Sonic 2, talking about how the art style of 3 was crap (when I thought it was the best in the series, even knowing how cheesy the Sonic attitude had become). Then there are a ton of people who like everything up to and including Adventure, when I see Adventure as a pile of garbage. 
 
My hypothesis is that it must have something to do with age and childhood nostalgia. There's some cutoff point where people became "too old" for Sonic. I've been trying to take a look back at older Sonic games so I can properly compare them to Sonic 4, and it's really gotten me to question how much of my love for the original games is based on the fact that I was a kid when I played them. I'm honestly not sure if my vision has been clouded by my childhood. All I know is that I can totally pick up any of those older Sonic games today and still enjoy playing them.

Sonic is pinball with a hedgehog in place of the ball.  They hadn't implemented that in their 3d sonic games, as far as I know.

How can you like Sonic Adventure and then go on to dislike the future games? That doesn't make ANY sense. Sonic 4 is pretty good, I just don't like the sluggish feel to it, other than that, it's straight up sonic 4 with ugly sonic.

When Adventure 2 hit, everything went downhill.
 
Hopin' for Colors.

" Sonic is pinball with a hedgehog in place of the ball.  They hadn't implemented that in their 3d sonic games, as far as I know. "

" How can you like Sonic Adventure and then go on to dislike the future games?  "

" When Adventure hit, everything went downhill. "

I actually really liked the Rush games (well, the first one at least), as well as the Advance games. Sonic's still solid on the portables, to me.

well maybe it isnt sonic games became horrible, everything else just became infinitely better

To be honest, I think Sonic fans are all wearing rose-tinted glasses. Those first games weren't all that amazing, they were just new. Going insanely fast, a pretty cool mascot? Great, fantastic, except you need more than that nowadays. So now when everyone was bitching "GIVE US BACK OLD SONIC YOU SUCKERS", you finally got it, and guess what? It's kind of middling. Because Sonic is a middling concept, and hasn't aged well over the last 15 years.

Putting in a story beyond "kill Robotnik" and characters beyond Sonic, also the furry fans.

Sonic 4 isn't that bad... People just have different expectations and Sonic Team just doesn't know what people want. They release 3D people hate it. They release 2D people hate it. They don't release and people go where he hell is the new Sonic. They do release and people go stop releasing Sonic. It's a vicious cycle.

Sonic started to go downhill when Sega thought Sonic was Mario and started making those adventure games. Only human to ever be included in a sonic game should be Robotnik (Eggman)

" @teh_pwnzorer said:

" Sonic is pinball with a hedgehog in place of the ball.  They hadn't implemented that in their 3d sonic games, as far as I know. "

Sonic Heroes had a fairly decent casino stage, if I remember correctly. I say 'decent', I was only 13 when I last played it. 

Other games came out that were better. Raising the acceptable gameplay standards of video games.

They never were good. I would go back and play Bonk's Adventure before playing a Sonic game again.

for me it got worse as they started pumping in more characters every game. 
they could have stopped with knuckles.

Swiftly, and without mercy.
 
Hope that answers it.

" Sonic has been terrible since day one.  Hold right.  Amazing game. "

  That's what Sonic's world should look like! Oh and YES, the NAME IS ROBOTNIK! "

They became horrible because Sonic Team (or whoeverthefuck) decided they should involve more than running really fast through loops and shit. Trying to shoehorn Sonic into other formats was a terrible idea.
 
Not that they were ever all that great to begin with. Sonic's probably the most overrated classic series in history.

They were all bad. You just didn't know any better.

Sonic was always bad from a mechanical standpoint . The character's primary attribute is speed. Going fast is fun. But going fast means you can't play the game, so any fast part is pretty much "whoooaaaahh" automatic rollercoaster eyecandy. This gets pretty old pretty fast, and any obstacle results in a dead stop which is not fun. During these slow bits you spend a great deal of time getting up to speed again which is not fun.
 
To solve this problem Sega could pull back the camera so you see more of what's ahead the faster you go, or shift the game into a 3D perspective, but doing that makes the feeling of speed less apparent and diminishes the main attraction of the game.
 
So it's a game that's only good when you aren't playing it. In order to fix this Sega just need to make sonic an incredibly agile character so he can traverse the world in a speedy, dynamic way instead of relying simply on raw straight-line speed, but that fundamentally changes the game and is clearly not something they are willing to look into. 
 
What attracted people to Sonic was playground rivalry between those who's mum got them a Genesis/MegaDrive, and those who's mum got them a SNES. It was marketing, flashy lights and good music. It was a highly polished game designed to arm Sega's platoons of fans against Nintendo's market dominance and to an eight-year-old it fucking worked. It worked for me, Sonic looked amazing on the face of it, but dig down into the actual gameplay and you find it was pretty much posturing and not much else.
 
Perhaps they should make a Sonic game where the hedgehog finally comes up against his ultimate enemy, the motor car, and flat-out loses in front of all his bawling shitty friends, who then get mauled by wolves.

It didn´t "become" it always was

I don't care what anyone says.  I absolutely love Sonic Unleashed

The people saying that Sonic games are just about holding right and jumping every once and a while have obviously never played past the first level in a Sonic game.

They got all buggy and unplayable. Plus Sega releases Sonic games like once a year with little regard for quality control.

When they just kept cramming in new characters and trying to create a real story, all hope was lost.  Shadow is a piece of shit character.  He uses guns?  What the fuck man?!?  Who gives a rats ass if hes a clone or there is some plot to destroy the world.  Really?  Whats the fucking story in Mario games?  Bowser stole my woman, I'ma kick his ass.  Thats all sonic needs.  Robotnik is doing some shit with these crystals?  Let me run real fast and bang on his hover chair. 
The first sonic adventure was great.  It has not held up because its problems are now pretty glaring.  The clipping and draw distance are a big one.  But the sonic only levels were all fucking sweet.  The story in that game was retarded.  But then they try to add rails and shit.  Nah uh son.  For a 3d sonic, go back to how Sonic Adventure did it and fix the issues.  No retard story please.