I've tried almost all guitar amplifier apps out there, and Amplitude is hands-down the best app available, coupled with an iRig. I enjoy it very much when I'm away from home. For my work I stay a lot in hotels, and Amplitude enables me to play my guitar there, without disturbing other hotel guests.
Many interesting effects are available, as is a nice selection of good sounding amps. I would have liked to see a stereo chorus effect, but unfortunately you can't have everything. The price is a bit on the steep side, but it's a great value for the money.