I just started playing the first Bioshock, and i know how late I am to the party, but holy shit... this game is unbelievable and I cant wait to finish it. I have never been so curious to go into every room and every corner of a game and want to discover so much. I'm only like 5 hours into the game, but I know now that I will be getting the second one 100% (GOW 3, Yakuza 3, Splinter Cell Conviction, Alan Wake, Red Dead Redemption, just bought Heavy Rain, and now Bioshock 2, all this year... holy shit I'm gonna be broke.) My only question to you guys, without spoilers, is the second one just as intense and satisfying as the first? Will it keep you curious and engaged like the first one? Sorry to go on for so long, but I friekin love this game! Thanks guys...

Main story in 2 is kinda meh for most of the game, as most people will tell you and I agree.
The ending parts are however much better then in Bio1, made it all worth it.

The beginning is definitely not not as fascinating as the first game's, but I still think the games story is good throughout.  The story really picks up steam towards the end, and I just adore the ending.  I think you'll like it, especially since you seem to really love the first one.  (Which I did as well)
 
Plus, the game play is sooooo much better.

By the sounds of it you'll like the second one. 
If you like finding audio logs, using weird weapons, fighting big daddies, researching guys to level em up, hacking robots n all that stuff: it all gets better in 2.
The only part that's not quite as good is probably the characters and the mystery, and it's a bit more video gamey.

I'm slowly playing the second one right now. And yeah, story did start off slow, but it's picking up now.
 
Gameplay is one hell of a lot better in the sequel as well :D