I played for a couple of hours last night.
Bioshock might have been my all-time favorite game, and it was certainly my favorite since the original Half-Life. I loved the writing and the way the story revealed itself, and the art direction and the whole Rapture aesthetic were amazing. On top of everything else I just found it fun as a shooter.
So far Bioshock 2 is good, but also a bit of a disappointment. The universe is the same, and still looks great, but in many ways the gameplay and levels don't feel as carefully planned to excite and startle. Being a Big Daddy is, in many ways, less interesting than being a guy who at least appears to be a total outsider to Rapture. Most significantly, the story, at least so far, seems much less coherent and inventive - it's more of a standard shooter where you are assigned tasks for what seem to be largely random reasons, and the core plotline just seems silly. It doesn't help that the voice acting is much worse than in the original.
All of that being said, I still love being in Rapture, and will definitely play this one to the end (for whatever reason shooters tend to be the only games I actually finish).