We have had two in our reef tank for a long time. My wife especially loves them, "Buster" and the "Big one with the hole in his shell".
They are great little creatures, and very entertaining when they take a swim, swimming upside down and getting knocked all over the place by the powerheads.
They do a good job of keeping the sand clean, and they spend most of the time buried in it. They will come out every few days and spend a few hours running around everywhere.
The down side to them is that if you have sensitive corals, they will retract, and may not like being walked upon (read:stumbled upon) by the crabs. If they wander into an anemone, which they will if you have one, they may become lunch.
Other then that, they don't require anything special, although I would recommend a large sand area, and temperatures not higher then 78 degrees.
Here's a photo of our pair.
