Though I can't speak for the iPod Touch, I ordered many skins from Decal Girl for my Nintendo DS and PSP about a year ago.
The skins stretch easily if you aren't careful in application, that is my only caveat. Otherwise, they do look pretty nice as far as customization goes. I mean, they still look like a vinyl skin on your device... but it's a cool way to make something your own!
Also notable - though they come off easily when you want them to, once one has been removed it is unusable again (stretches out of proportion).