Having just finished Bioshock 2 (I tucked it away when I first got it during finals, and then life came along and I just now got to it) I just wanted to take a second to give my impressions.
Overall? A great ride. Not awesome like the first (since the first was so novel that I, as well as many gamers were fascinated by it). They did add some things to the system, notable multi-user play, but for me this was a one man (or Big Daddy) adventure. I finished the game after about 20 hours of gameplay. I did it the nice way, by not "harvesting" the Little Sisters (comes back to my morality, I have a hard time tolerating, even in gameplay, the killing of little girls, even if the Little Sisters are some of the most creepy girls you will ever encounter).
Mainly? It felt like more of the same. The only real differences were the fact that you played as the Big Daddy (which meant you started out with the honking huge drill on your arm), the very novel bit where you got to play as a Little Sister (the whole world is clouded and unreal. I did like how they play off the Angel thing was pretty cool), and the novel new opponent of the Big Sisters.
Did I enjoy my return to Rapture? Yes, as evidenced by the some 20 hours of gameplay I devoted to this game. Was it as good as the original? Probably, but somewhat less novel, so in the end the original probably comes out ahead.