IMO, they are completely worthless and borderline harmful in a main aquarium setting. In a hospital tank setting or in a large Fish Store where many tanks are using the same water they can be helpful but they won't solve any problems by themselves. I would never tell anyone to use one to fight off "ich" in a display tank.. that would be futile and pointless because it simply isn't possible to kill all the ich in a tank with a UV sterilizer... but while you are killing some ich with it your also killing a lot of good things in your tank as well. Only the free floating stage of ich is affected by UV sterilizers, hense why UV won't solve any ich problems, it only assists during one stage of their life cycle, and even during that stage UV is not 100% effective.
The best method to avoid ich in the display is to hospital/quarantine all new fish, especially tangs, for at least 4 weeks, preferably 6-8 weeks to be really sure. If your fish is ich free in a hospital tank after 8 weeks then there is little to no chance of that fish introducing ich into your tank.
Just my opinion on UV sterilizers.. others may have different opinions. The more opinions you get the more information you have in hand to decide if it is something you wish to pursue on your tank.