We "adopted" a tuxedo urchin from a fellow BRSer about a month ago. He is an amazing creature. He climbs the glass, over live rock, on top of powerheads, over some mushroom corals, and he leaves a trail of super clean rock in his path. He carries bit and pieces of coral and tank debris in his spines, which I understand means he is a happy critter. He's great fun to watch and you'll never know where he is likely to appear next. I would guess that if you had a small tank with lots of delicate frags here and there, he might be tipping them over or moving them out of his way during his travels, but from our perspective, he's an interesting addition to our tank.
Andrew