I'm holding off for now, and will only get it if the reviews are really good.
Personally, I think BioShock stood just fine on its own and didn't need a sequel. For such an "artistic" game, it's pretty clear that the sequel is a cash-in. I obviously haven't played it, but the idea of a multiplayer BioShock seems contrived. The only somewhat legitimate reason for a sequel is to expand more on the city of Rapture, even though if you listened to all the audio logs, you'd know what you needed to already. I'll probably take shit for this comment from people who just want more, more, and more, but I think I'm being pretty rational.