Well, on the one hand, over several replays, I've found myself VERY bored with the opening levels, since they're pretty much exactly like the original. But on the other, the story, the themes, the option to kill or be merciful to the people you meet and having that affect the end of the game, AUGUSTUS SINCLAR (and Augustus Sinclair's voiiiiceeee :3), the themes of fatherhood and mercy and humanity, and oh sweet jesus, the level where you're a Little Sister, which was pretty much one of the most unexpected things I'd played in a long, long time, and it was delightful.
It wasn't a perfect game. Killing Little Sisters flat-out didn't make sense from a thematic standpoint, and why on earth were you given the option to only save SOME of the boss characters you met? Some of the levels were far, far too similar to the original, and I agree that the idea of multiple, easily killable Big Sisters made them far less scary. (Although the bits where she's chasing you and making the televisions explode and the screen shake were incredible.)
I also think that it took the theme from the original Bioshock (If given the opportunity to do the right thing when you're in deep shit and there's no light at the end of the tunnel, would you do it?) and expanded on it in a really effective way. I very much enjoyed it, and have rented it more than once. Flawed, but still worthwhile.