I had a nice little monster that went after monti/acro polyps and also my open brain. I know some people (not me personally) have had them go after acans/other LPS like them.
As one of my old LFS owners used to tell me, "there's about a 50/50 chance"
That said, I'd definitely say bad for a reef tank. However if the tank is SPS dominated and well matured maybe not so much, since the damage would be minimized (hopefully) to any one coral. However I wouldn't trust it if there were any tasty poofy lps that were in the tank.
I had one in my softy tank some time ago, and didn't notice anything affected by it but then again I kept mine well fed.
Picked one up from Neptune from their last shipment. Small little guy at 1.75" or so.
Figured that I'd give it a go (50/50 like Mike said) after seeing the fat ones at DPV. Seems like a model citizen so far and hasn't touched any of the LPS. It does get fed quite a bit like the rest of the fish however.