Some say they're bad, some say they aren't. IMO I haven't ever had problems with them, even in my current tank. But as with everything people have had different experiences.
If you use the 'search' function up at the top of the home page and look up asterina, you will find a LOT of info on them. There have been a number of excellent recent threads regarding them.
I had them, they were fine untill they started reproducing to the point where they were so large in number that they were sitting on corals and smothering them.
I got one about a year ago on a frag I bought fomr a member on this site. I probably have 300+ of em now, haha! For a while now I thought about buying and invert that eats starfish but they seem harmless and have caused me no problems. I guess it just depends if you like em or not. They seem to do a good job on cleaning up some of my algae though.
i find that hard to believe but i guess its possible.. i did hear that they are coralline eaters, but for most ppl its not really an issue that bothers, me included