I agree with everything here. I can't figure out what everybody's desire is to get them. If you have to get one, I would only keep one or two. I've got 4 in a 38 and I hate them. I thought they'd be great at first so I paid the 9 bucks a piece for them.
What's wrong with them? Well, as was said earlier, they are opportunistic scavengers. They are fast, mobile and strong. If you try to feed a coral, they will SMOTHER it immediately so the coral closes up, and rip away any food they can find. If you try to appease them with food to begin with, they finish it up and rob the coral anyway because some coral can't ingest food fast enough. Basically, if you get peppermints, you can forget about doing any successful target feeding.
I saw one of my serpent stars grab a piece of silverside one time and start eating it. A peppermint shrimp (that had already been fed mind you) flew over almost immediately to grab the food away. I thought, " No way. The serpent star is too heavy and too strong." Well, the shrimp started ripping and yanking on the silverside like a dog pulling on a rope and after about 5 minutes of this, ripped the silverside away and ran off with it.
I would not add them to a new tank unless I had a bad aiptasia problem and then no more than two.
Oh, BTW, I couldn't even add lettuce nudis to the tank. I dropped one in and the shrimp thought it was food. One grabbed it before it even hit a rock. The other 3 rushed over and immediately ripped it apart and ate it. The other 2 nudis dissappeared by the next day.