I have to give it a solid "Meh." Rented a couple days, played a couple hours, got tired of it and went back to MLB The Show. I liked the original Bioshock, though I'm starting to come to the conclusion that FPS games just aren't my thing.
One thought I'm having is that I like the idea... You play a Big Daddy. Nice immersion while you're stomping around smashing random crap with your drill-hand. Horrible immersion when a splicer with a saturday night special takes you out with a couple shots. I mean, sure, I'm a weakened state Big Daddy... but what a wussbag!
In the end, it's pretty slick but not my thing. Like the improvements overall, but for me the allure of the first Bioshock was the environment... and this environment has been done before.