From what I"ve heard it's hit and miss. Some people have them and never have any problems, some swear they're satan incarnate. I'm planning on getting a flame angel, and I realize I may wind up having to pull it out. But from what I've heard, if you keep them well fed they're less likely to engage in any bad behavior.
In time they all with pick and when they do, they will usually eat the whole coral that they're picking on. So you need to be prepared for that. Once you put one in a tank you need to make sure you can remove it otherwise you'll have to live with it picking on certain corals. If you can live with that, as some people can, then you should add one and not worry about it.