Yes. I was using a small 8w "LifeGuard" unit on my 35 gal Q tank (the units sold by Pentair Aquatics). It was given to me - so the price was right!
To pump the water through it, I hooked it up inline with a small Fluval canister filter. The Fluval was on its last legs so the flow rate was very, very slow. How much of an impact it had is difficult to say. I did have one fish go through Q with ich that cleared up, but I used a variety of approaches - in addition to the UV. I've since taken it out as I replaced the Fluval with a Magnum and am not sure if I will put the UV back in-line.
I think UV system are beneficial for reducing infectious agents - in particular bacteria and infectious stages of Ich and similar protozoans. If I had a serious outbreak in my main display, I would seriously consider the use of UV as a part of a strategy to beat the parasite back where copper would not be an option. However, they are not fool proof and effectiveness depends on many factors alluded to by others (wattage of bulbs, age of bulbs (aka intensity), flow rate (aka contact time). It's not surprising that many large commercial aquaculture operations use them effeectivel to manage disease (or more correctly, to reduce the infectious burden on their fish).