UVs are debatable, and there are good arguments for them and against them. In a fish only tank, I think think they are GREAT. However when you get into corals, ScubaDoo can give some good arguments as to why you should reconsider them.
Personally, I believe in UV sterilizers but they don't really prevent ich, they help by splitting the DNA of existing ich in your tank, thus the population is reduced.
When you are dealing with ocean life, you run the potential of introducing ich each time you put a fish in the tank, which is why you should QT all fish.
The important thing to remember is to get a UV that is rated higher than your tank requirements and to get the slowest Maxi-Jet you can find so the water has ample time to be exposed to the light. Too much water flow and you are basically just burning electricity.
I purchased one for each of my tanks after my first ich outbreak and haven't had problems since.
Another new tank is being prepared and I'll be using another UV myself.
Denise M.