I had one for several months. At first, he was a cool scavanger. Then he killed a hermit crab. I let it go as I had tons of them.
Then he got larger and killed a peppermint shrimp. That was about it. Before I could get him out that afternoon, I saw him corner a juvenile clown and try grabbing him. He went back to the pet store a few minutes later.
Soem have luck with them, many don't. I feel that they are reef safe when small.....but small crabs grow up quickly.