Just on hearsay some people seem to swear by them, some don't.
I've got them on half of my tanks at work, half without, and see no difference whatsoever. They still get ich from time to time, the algae still needs wiping almost daily.
But to be fair to UVs, the units at work are straight tubes only about 12" long or so. What I've heard is that when the water is in contact with the light for that tiny bit of time, probably only exposed to the light for seconds at the VERY most, no, it won't do any good. The best ones are the corkscrew shaped ones, the more twisted and long they are the better, so that the water is in contact with the light for a longer period of time. The longer the water is in contact with the light the more it will kill, as I understand it.