The problem with this game has always been that Bioshock didn't need a sequel; everything was tied up beautifully and we saw pretty much everything that we wanted to see in Rapture. So when this game was announced, I was immediately skeptical of how they'd make it different from the first. Answer: they didn't.
Okay, so you're a Big Daddy now but that only really means that you have a drill instead of a wrench and you can breathe underwater in some sequences. Saving Little Sisters now forces you to carry them around the map and fight hordes of enemies, but you can get all of the plasmids and slots from just harvesting them, so why bother? Also, you can dual wield plasmids and weapons, but it doesn't really change the dynamic much at all. Therefore, the obvious reason why they went the Big Daddy route was so that the story could center around your Delta status and the semantics behind the Big Daddy/Little Sister connection.
And story-wise, it does a pretty decent job, but fails pretty badly at explaining it correctly. You'll get some really cool Bioshock moments with insane people and the like, but how it fits into the main story is only ever really explained by hard to understand tape recorders lying around, or easy to miss sentences uttered by the characters. I had to go read Wikipedia after beating the game to understand what had just happened.
Still, generally the game is pretty fun and features some great environments, despite a loss of steam in the middle sections. Overall it's a game you'll forget about after finishing, but will enjoy while it lasts... Mainly for the Bioshock nostalgia.
In a sentence:
Almost exactly the same game as Bioshock, with some minor changes and a less interesting story, but also great environments and scripted events.