Are any self-cleaning cat litter boxes any good, or worth the money?

Does anyone have any experience with self-cleaning cat litter boxes? I'm curious if any particular model of self-cleaning litter box is any good. We now have 4 cats and it would be nice to not have to clean litter boxes manually 1-2 times every day.

Do they separate pee/poop from litter well? Are cats afraid to use them? Do they stink more than regular litter boxes because pee/poop are in them for longer periods? Are they a hassle to clean? Do you have to buy propietary supplies (custom litter? special trays?)?

Thanks for your input.