they can eat corals, zoa's especially, so reef may not be too good, and they may need target feeding of lettuce or sea weed (just pick them up and put them on a piece of it) so they may be too much work for some
I have 2 Money Coweries in my reef tank, and they're great. They really mow down the algea. Neither go near my zoas, though I do occasionally see them on my Star polyps (no damage though). Don't freak teh first time you see them with their mantles fully extended, as they'll look brown and fuzzy.
I've heard some people say that they are nocturnal, but mine are out 24/7.
