I haven't used one, but know people with multiple cats that absolutely swear by it. I don't remember Tony having any problems with his cats getting used to it, but I guess it just depends on the cat. I would also think it would depend on where the litter box is. The Littermaid has a 10 minute timer (I think) so if the litterbox isn't where kitty is 10 minutes after they go they wouldn't even know the difference. There is also a motion sensor that resets that 10 minute timer so it doesn't scoop up kitty!
I think it's a great idea, but a little on the pricey side for me right now. (We actually put it on our wedding registry, and for some reason no one thought getting us a litter box for a wedding gift was appropriate! LOL)