Bioshock 2 does not live up to its predecessor, unless you are a hardcore fan, I suggest renting the game first.
GRAPHICS: Nice lighting and textures for the most part. Good animations and character models, too. Everything actually looks nice, except for the same old bland Rapture we already saw for 14-18 hours in the first game. Not much was added or improved.
GAMEPLAY: Basically the same as the first. This time around, there are new plasmids, guns, tonics, and dual wielding has been introduced. Dual wielding is very helpful with experimentation. New enemies were also added, no big deal. Hacking returns, except now instead of pausing the game to do a minigame, you can now shoot the target from a distance and hack it by doing this "slide and stop" minigame. Killing people with your drill is immensely satisfying, and overall, you seem like a very powerful brute. You can now pick up Little Sisters and bring them to bodies, which grants you ADAM. However, during the ADAM extraction, you have to defend her by setting up a defensive perimeter. Sometimes this gets repetitive. A part of the game that was exaggerated like crazy was the "step out of Rapture and into the ocean" idea. Total lie. You get to leave maybe two or three times in the game, and you get to walk around in a very tight, small area. The scenery is cool and the ADAM slugs are always good to collect (10+ ADAM each), but overall, it was a feature that had flash, and no substance. Sometimes there is lag in the game, which took away the smoothness of combat. The game lasted me about 10 hours or so, give or take an hour.
AUDIO/SOUNDTRACK: During combat involving more than 2 splicers, the sounds skip and drop. Basically, the sounds lag, and make the game sound like crap. This is hard to ignore, the developers could've spent another month or so polishing up the audio department. Voiceacting is good, but the soundtrack is forgettable. A failed imitation of the first game. Close, but no cigar.
ONLINE: There is online in Bioshock 2. Its fun for the first 5 or 6 hours, but then it gets a little boring. Its the same as the single player. As you level up by killing people, hacking, and collecting ADAM vials, you unlock new plasmids, tonics, guns, and gun attachments. This is all pretty cool, and you can even customize your character. Being a Big Daddy is neat, but not that fun. You just beat the single player as a Big Daddy. Killing other players online is sometimes hard and unfair. Its kind've like Black Ops or Modern Warfare 2: You shoot the guy 4 times with a shotgun, he lives, and kills you with a few pistol shots.
OVERALL
Pros: Fun online for the first few hours, nice new perks and tonics, being a Big Daddy is awesome
Cons: Audio is poor, online gets boring, Rapture is the same