I've never used a UV sterilizer, and since my tank is pretty stable, I don't see the need for one on a regular basis. I think I miht use one if I had a CRAZY algae problem, but I think it's more of a cover for an underlying problem. The could be handy, however, if you fixed the problem and then use the UV to clean up everything.
That being said, I've never researched them much, so maybe someone else will provide a counter point.
Matt