I've got UDK installed, and I've had Unity installed before. Couldn't work out how to do anything with either of them.
If I could guarentee I'd actually get around to producing something for the i-devices, I really would take the plunge with some of my cash. Given all the other stuff that crops up all the time, I'm just not in a position to really give it a whirl.
Who knows, maybe someday soon