The multiplayer can be fun, but it can easily come off as gimmicky. The designers borrowed a lot of elements from other games, but gamers recognized the world of Rapture for its fresh art direction and storytelling, not for following the standard. It's not fresh or new, just a different coat of paint. BioShock 2's multiplayer doesn't bring anything new, but it's a nice distraction from the campaign.
If you're playing BioShock 2 for its multiplayer and not its narrative, then you're doing it wrong. Think of it as nice-to-have bonus materials in the pocket of a DVD you've been waiting for.