For the most part what I have heard is that the majority of flame angels are fine with reefs, but you can occasionally get a bad one. However, I just started a thread yesterday about keeping a flame in my 30 gallon tank and was told that a 30 isn't big enough to keep a flame happy and that you would be better off going with a pygmy angel instead if you still want an angel. They aren't as good looking IMO, but I'd rather have a happy not as good looking fish than a stressed and dying flame.
-Scott