From what I have heard, hyposalinty is the best way to cure ich, and it has worked for me. UV Sterilizers will get most (don't think it is all, but I may be wrong) of the free swimming Ich. Once it is on the fish a UV Sterilizer will not get rid of it. So in reality, UV Sterilizers are more preventative and may keep your outbreak controlled (if that is possible) by killing the swimming parasites. It will not cure ich on your fish though. A UV Sterilizer will also kill some of the beneficial bacteria etc. in your system. That is a price you have to pay. It doesn't pick and choose what it "sterilizes".
I can see the benefits of having one, and I have contemplated getting one when I have the extra cash. It will not get rid of Ich on your fish though.
I am not an expert on Sterilizers so if anyone else know elsewise please speak up.