I just finished this game a few days ago.
I liked it, but while Brotherhood was praised for being much more than an addition to an existing game, this one really felt like "AC 2.6" (if Brotherhood is 2.5, I suppose). The single player campaign was too short, and the complete absence of the glyph puzzles that were a fantastic diversion in AC2 (and less so in ACB) was a big disappointment. The Desmond puzzles were a pale, pathetic imitation of the old puzzles, which had you scrutinizing the backgrounds of the images and Googling stuff and breaking codes. So much fun, and as far as I can tell, there's nothing like it this time.
Seeing Ezio's story, and Altair's, brought to a close made the game worth playing, though. When Brotherhood came out, I remember thinking that Ezio's story could have been complete with AC2 and that I would have preferred a new protagonist, and then was really upset with the events of ACB's first few chapters, but then when that game finished, it felt too open-ended, so the conclusion in this game felt good.
Constantinople offered quite a few great tall buildings to scale, and some great views, and wandering through yet another lovingly crafted city is always a pleasure. I liked the mixture of Turkish and Greek that you heard from all the NPCs. Ezio's pronunciation of Turkish words is surprisingly good, to my amateur ears.
Multiplayer is a lot more fun this time around. I can't remember the details (correct me if I'm wrong), but in ACB there were few options for the newcomer to ease himself into the game. Basic deathmatches had level-50 juggernauts crushing you with not just their experience and skill, but also with abilities that you wouldn't be getting for a very long time. It added up to a miserable experience in which the noob had no chance whatsoever and would be killed again and again.
In this game, though, "Simple Deathmatch" removes all those abilities and makes the battlefield smaller, so a low-level player has more of a sporting chance. I'm still honing my skills in Simple Deathmatch despite being level 19; I like it because most of the other players have levels from about 1-30, with the occasional level-50 god slumming it.
"Artifact Assault" is my favorite multiplayer mode, though. Noobs can get into it just by hanging around their own artifact and defending it, but the real fun is when you try to grab the enemy's artifact and take it back to your base. The rush of successfully scoring -- I won't forget my first "goal" any time soon! -- beats anything in the single player game!