Mantis shrimp get a bad rap for no real reason. If mantids were everything they are built up to be in regards to their destructive power we'd be sending them into the hills of Afganistan already...
Like any creature one would put into a reef tank, you must first know what their diet and habitat need to be.
Mantis shrimp fall into two generic categories: Smashers and Spearers.
Spearers eat fish. They have specially adapted appendages and quick reflexs they use to dart out of their burrows and stab/spear fish. In the wild these live predominantly in burrows dug into muddy sand-bottoms. Much less commonly found for sale or as a hitchhiker than smashers.
Smashers eat crustaceans/shellfish. They use their specially adapted appendages to crush, crack open and bludgeon their prey (snails, hermits, crabs and shrimp). Smashers are much more commonly found in aquaria for sale and as hitchhikers. They will, for the most part, live in holes burrowed into LR or will build burrows from rubble.
I only have personal experience with G. smithii (smasher). Mine LOVES nassarius snails when I leave him to hunt (which I do more often then not). I also feed him mysis shrimp straight from the turket baster. He'll grudgingly eat shrimp pellets.
So in a reef with fish a spearer might be a bad choice (fish are generally more costly than inverts), but as long as you know you'll be replacing snails and hermits (which if you have hermits and snails mixed you'll be replacing snails anyway) a smasher needn't be considered a pest any more than any other inhabitant of your tank that may eat another.
Mine has not started any genocidal campaigns (except for the Nassarius - and then only when hungry). He has not built anyweapons of mass destruction. Neither has he begun making demands from his side of the glass for expensive corals or fish to throw around and abuse...
Hope that helps,
