I had one, hated him after a bit, he kept taking my loose frags and took them into his cave.
He's amazing at keeping the sand bed clean - when I got him out holly cow red slime everywhere.
I had him solo, he bugged the wrasses i have, dottyback, and other similar fish but never killed anything.
I've moved him from my reef tank to my breeding tank because we like him but was sick of him hiding frags and burying things with sand as he filtered it through his gills.