I've done a few tracks running an improv Korg iMS-20 session into Garageband or ProTools.
Just ordered the iRig to use on some sideman type gigs for songwriters where I don't want to blow out a listening room cranking an amp. I'll just run it into a DI and let the soundman give a more controlled stereo mix. Certainly won't use it all the time, but it will be a great option depending on the room.
the iRig is pretty neat, especially if I am just sitting in my apartment and just want to play with some headphones on, I dont have to plug all my shit in.
a neat little app I downloaded is Pro Keys. It has 2 sets of keys on top of each other, and like any keyboard, you can choose your different instruments/effects (grand piano, rhodes, b3, synth lead, mallets, etc) but it also has 8 or so drum pads. You can record tracks and loop them to play over them, or you can upload songs from your itunes library to play over. Pretty sure these sick beats im writin are gonna change the world

just something cool to mess around on, prob would never use it live, even though with the iRig, I could.