I have played and own all Halo games. minus ODST. Have beaten the campaign's for all of them and have played the multiplayer for all of them, however after a few days I often grew bored with Halo and could never quite get into it. 
 
I finally bought an Xbox 360 on October 15th after years of being away from gaming and along with it I bought Halo 3 and Gears of War and by the 25th had grown bored with Halo. I beat the campaign quickly, earned around 100 EXP points online, played ranked and social playlists extensively, however I still felt unfulfilled. I had gotten good at the game quickly and was dominating Lone Wolves 9 out of 10 times. But I still lost interest really quickly. The team matches were more like lone wolve because nobody worked together.
 
So after hearing that Halo Reach had all these new features such as weapon loadouts and powers unique to each loadout (sprint, jetpack etc..), as well as a new graphics engine, I went ahead and bought it (yesterday afternoon). Brought it home, popped in the disc and started with the campaign. So far so good, graphics are solid, the DMR rocks, the bloom feature makes the game more skill based as opposed to the recoil-less weapons Halo 3 had and the 1st half of the campaign was pretty solid as well. Then I moved onto the multiplayer. Armor customization has improved, theres no longer separate play lists, just one large competitive playlist as well as an arena playlist and a coop playlist. All the old playlist modes made it into reach minus lone wolves. However Invasion is a nice addition and I enjoyed it. Rumble Pit is quite similar to lone wolve's as well.
 
I remember playing Halo 3, thinking to myself: 
 
Why cant I choose my weapon before I spawn?  
Why cant I sprint? 
Why cant the battle rifle fire 1 shot at a time instead of 3? 
 
And apparently Bungie was thinking the same thing. The powers are fun as well. The jetpack is solid, the bubble shield is nice, sprint works well, armor lock has its uses. The DMR is A VERY VERY SOLID weapon. Probably the only Halo weapon I have ever liked. Its the most skill based weapon that Halo has ever had and I love it.
 
Coming from games like Battlefield 2, Bad Company 2 and a bit of Call of Duty, Reach has really won me over. I haven't been able to put the controller down for the past 8 hours. Something I wouldnt have done with a previous Halo game unless someone had a gun to my head.
 
By the way...
 
ARMOR LOCKING WHILE A VEHICLE IS TRYING TO RUN YOU OVER IS SO ****ING EPIC I CAN'T BEGIN TO EXPLAIN. Its an instant kill 90% of the time.

I liked the old way Halo was a little more, that's just how Halo was.  Now it seems like their taking some ideas from COD or other class based FPS for sales.

I played Halo 1 and 2 so for me its 1 thats the best I want to play Halo 3 4 and 5 badly too

if you are the kinda guy who pay attention to story more than the multiplayer
you really should try out ODST, IF you didn't got spoiled on the story yet...