My local shop says that they are good for fish only tanks but can zap out nutrients that a reef tank needs...but if you have a reef with tangs in it you will have to find an alternative way to control the ick outbreaks...Other shops in town seem to agree with this as do the few reef carers I have talked to at Epcot and Seaworld
Put it this way...we had a really bad ick outbreak. Our lion looked horrible. We went and bought a UV sterilizer...next day...he was 200% better. I think it's a great investment. Our lfs said we shouldn't run it constantly though...just when we noticed signs of infection, etc.
UV isn't gonna rid ick once its on a fish. So if it dissapeared over night it wasn't from the uv. UV only kills ick when it goes through the light. They usually stay on the fish. They fall into the substrate. UV are most effective prevening ick going from tank to tank when having multiple tanks. UV aren't very effictive. Most ppl buy under rated once also. if you tank has ick you are stressing it= tanks to small over crowded poor water, aggression ect.. Most ppl over crowd there tanks=always have ick problems. Better than a uv is qtank and low stocking with compatible fish. IME