I've used them in the past, and they DO wreak havoc on FW's...in fact, FW's see them and try to RUN from them. The only trouble is they starve once the FW's are gone, so if you can pass them on to another aquarist, do so.
It's my understanding that their diet is the rust-colored flatworm or red planaria. Since this thread is in the SPS discussion, it needs to be pointed out that this nudibranch doesn't feed on AEFW.