I do not use one because I believe that the Cons outwieght the pros here.
UV's are using ultra violet radiation which in essence zaps micro organisms (emphisis on micro organisims not just ick) by destrying the nucleus of the cells. This will also kill benifical microbes, larvea and planktonic stages of your pods, worms, and many other critters that live in your system and are very necessary to the health of the system. This means less food for your filter feeding corals and such. Will it kill the parisites that could attack your fish, sure, as well as kill nuciance alge spores, also your coralline spores. This just isn't something that I would like in either of my reef tanks. Why would anyone want to kill their bacteria? Far better to invest in a Q-tank.
Now thats my opinon and I'm sticking to it.
Thomas