I've used it on guitar amps, percussion, and even some vocals. It's a pretty cool mic, very even sound but with that typical upper-mid bump you find in a lot of BLUE stuff.
I haven't used it much on amps since I got an e609 though, but I'm keeping it around anyway
I have one and use it often on amps. It's really easy to dial in because the proximity effect is gradual. There's a lot of usable sounds from 0-12" away. Some other mics can be all or nothing. You sometimes have to get creative with the mic stand though since there isn't much travel on the mount. If you don't already have a sm57, get that first.
We used it at school for micing a guitar cab, along with an SM57, and a AKG 414. it sounded good, but the 414 blew it away. but its a good mic for the $$$