Monolith's downloadable multiplayer shooter, Gotham City Impostors, will run you $15 when it launches in winter of 2012. So that'll give you plenty of time to explore Batman: Arkham City before the first-person title hits Xbox Live Arcade and PlayStation Network.
I got my first look at the game earlier this year, and was completely taken off guard at first. A team-based multiplayer first-person shooter set in the Batman universe? Maybe Monolith has lost it's mind, but it's backing up the premise: wild wannabe Batmen (and women) versus wannabe Clown Princes (and Princesses) of crime duking it out in 12-player FPS battles.
Gotham City Impostors definitely had a goofy vibe, one I think does a pretty good job towards helping to set it apart from the dozens of other shooters on the market. For more, check out my hands-on preview.