Video playback is an interesting use case for AirPlay and the iPad. If you don't have mirroring enabled, you can actually start playing a movie on the iPad, have it stream to your TV via the Apple TV, and go about using your iPad as if nothing else was happening. Most apps will allow you to stream video in the background without interrupting, however some games (e.g. GTA 3, Infinity Blade 2) and some apps (e.g. iMovie) will insist on streaming their UI to your Apple TV instead.
Although iOS and the iPad don't do a great job of promoting multi-user experiences, using AirPlay to push video to a TV wirelessly is an exception. If you frequently load your iPad up with movies you can use it to keep others entertained while you either get work done or just goof around on your iPad at the same time. It's a great fit for families where people want to do two different things. If you do put a lot of movies on your iPhone/iPad, the 3rd generation Apple TV is probably a must buy for this reason alone.
