Got my hands on an iRig device so I could plugin my electric guitar. It's pretty neat and with Garageband, you almost have unlimited options as GB has many simulated amps in there. I did tried some other apps but I must say that GarageBand is the best for that device.
Basically, iRig is just an adaptor to connect your 1/4 plug into the 1/8 socket hole of the iPad. To hear what ever you play, you must use headphones since plugging an external device is considered as using the usual headphones. I was a bit disappointed to discover that is it impossible to AirPlay the output to your AppleTV as I was considering using it as my amps thru the TV sound system. In GarageBand for example, when you activate AirPlay, you get a warning that "Input" will be disabled while outputting sound to the AirPlay device. A small annoyance but it would have been fun.
Otherwise, this is a great way to play some electric guitar without disturbing the neighborhood, at least, if you do not sing at the same time...