UMP + silencer = surprise buttsex for the enemy and many a smugface for you. Win quite a bit of head-on encounters with it.
I don't know what is is exactly, but it is surprisingly accurate (also due to the low fire rate) and hits quite hard for an SMG. Kudos to Urban Terror, which gave me the idea to actually try the gun in BF3.
edit: I remember now it suffers from the small mag size though, however it never was a real option over other things, unless you decide to play SMG recon