I've had one for about 3 years...he was a hithhiker on some live rock. When I saw him the first time he was only about 2 inches long and I thought I'd lose every fish in the tank. Tried trapping him, without success, so I figured I'd have to take the consequences. Anyway he hasn't harmed any of the fish but I have lost the occasional snail and can't keep any crabs.
I pulled my tank down about 5 weeks ago, anyway to cut a long story short I found the mantis, now close to 4 inches long and transferred him with the 3 fish I intend setting up in my new 180 gal reef into a temporary 20 gal Q Tank. He is a real personality..he's set himself up in a barnacle cluster, plugging the holes with small snail shells and at feeding time he pops his head out to see whats for dinner. The fish wrestle with the food and ol'torpedo comes up from the bottom and grabs the food and scuttles away. The other fish aren't in the least bothered by him...one of them is an Orchid dottyback smaller than ol' torpedo.
I'm gonna keep him in the Q Tank, no way would I get rid of him. He looks great, purple claws and black body...he moves really quick and studies my every move when I go near the tank. They are worth their own tank. People are blown away by his antics.