So here' a quick rundown.  
I'm a huge Castlevania enthusiast (finished all of the GBA/DS Games).
Bought this game yesterday at around 5/6 in the afternoon. 
Note that I have only played Single Player (Xbox Silver Account). 
Game is absolutely stunning and engaging, presentation, soundtrack are fabulous and loot collecting is very addicting. 
Before I knew, I have finished the game...at around 6 in the morning, that's how gripping it is. 
Last form of the last boss is no joke if you're going alone (last incarnation), it wasn't in Rondo of Blood nor SOTN (flashback). 
So yeah, as a single player game (and I assume multiplayer) game shines, don't let some of the reviews discourage you, it's a robust adventure game that has whole lot of replayability  that made other Castlevania games so enjoyable to go through multiple times. 
Recommended!

I haven't been disappointed in the solo game, either. I had to work on the day of HD's launch, and I was about to join a group when I noticed that they were playing chapter five. Two things went through my mind at that point. First off, I was thinking that I'd probably get thoroughly PWN'd if I attempted playing chapter five from the start. Secondly, I realized that I wanted to finish the game in solo mode before playing with other people. After all, I just wouldn't have the same sense of accomplishment if other people were doing most of the work (and they would've definitely been doing all of the work, because I had horrible gear).

I just played for 4 hours on hard mode with some guys from America, it was awesome.
We got to the 5th chapter's boss, and died. And then died some more!
Didn't matter though, I had a blast.

I really, really enjoy playing this with both friends and random dudes on XBL, but yeah, I sink a LOT of hours in the single player too.  Sometimes you just wanna kick back and level up some abilities, or do map runs to try and collect souls/spells off a specific monster.  Still haven't beaten the game.   Drac is giving me some trouble, but I think I've got his patterns down, so I'm gonna take a run at him solo today.  I've tried with a couple different set ups of people in multiplayer, both friends and randoms, and it usually just turns into a gigantic mess.  Half the team getting charmed, the other half just running into the middle all willy nilly and getting ho-slapped around...  I figure my chances at beating him might actually be better by myself.

Play it with a fight stick for a new experience. Don't forget to set your default zoom level since you won't have access to the right stick.

I agree.  It definitely isn't meant to be the sweeping adventure that the handheld games were or that Symphony of the Night was, but it can be very enjoyable as a single-player. 
I like playing levels single-player to learn the layout of the map, learn enemy patterns, and to challenge myself to make it deep into levels designed to be taken on by six players.  It's a bit grind-y, but that makes it challenging.  You hit a level, die.  Get better gear, die.  Find a new route to the boss, die.  Put it all together and overcome.  It's just a great feeling. 
Not to mention, I love the fact that you have five characters to fiddle around with.

I'm glad to here some positive feedback on the single player. I've been on the fence about buying this due to the reviews bashing the solo mode and my lack of a gold account, but i think you guys have swayed me. I'll be picking this up tonight!