To make it short, it's a great game, but it's pretty much the same game as System Shock 2, except toned down a bit to make it easier for consoles players...
That sounds bad, but it still makes it one of the best FPS released recently. 

haven't finished it yet, but i don't really think you can call it a 'spritual successor' to systemshock 2.

it's pretty much just a shooter. there's a lot of variety with different plasmids, but the effective ones are really just the direct damage ones. bees are handy as CC, because guys just tend to stand around yelling "i'm covered in bees!".

it's like they started off making it cool like SS2, then at the last minute decided to really dumb it down and turned it into a shooter.

that's not to say this isn't a great game though. graphics are top notch, with a consistent theme throughout and really well thought out level design.

movement, thankfully is very responsive. this is one place where i'm glad they deviated from SS2. i didn't like the floaty movement in SS2, but in bioshock it's nothing like that. you still move pretty slow compared to HL2 or Q4, and there's no sprint (or walk?? o.0) but it fits with the game.

i don't know if it was the publisher that wanted the game dumbed down, or if they did it because of the xbox (damned consoles! >_<) but to me, it really brings the game down a notch.
not having an inventory is just plain stupid, and having to go to a gene bank just to check what gene tonics i have equiped (and what tonics i have stored) is also stupid. weapons don't degrade (which would have made a LOT more sense in bioshock than it did in SS2) so you can pretty much use your stronger weapons with impunity.
the ammo conservation and item scanvenging is gone from SS2. I'm routinely full on money, and there are vending machines all over the place. i seem to ALWAYS be full on flame thrower ammo, same with freezing ammo (i save eletric gel for daddies).
personally, i find the electric gel overpowered. it makes big daddies a joke as it stuns them for a half second or so and it's a constant stream and it does a fuckton of damage. couple that with lightning plasmid and electric buckshot and you pretty much perma stun a big daddy from 100% to 0%. not only that, but daddies are also affected by freezing plasmid, so even if you're out of electric buck or gel, you can still effectively CC them making fighting 2 at the same time entirely doable.

on the plus side, the story is really good with interesting twists and such. voice acting is just as good as SS2, and models for main characters are awesome-- much better than SS2 (animation in particular is very fluid and natural).

anyway, probably more to be said, but i'm hungry. :)