I had always been very skeptical myself. They are capable of killing basically anything that passes in front of the UV light. However, there are so many variables involved like how many watts is being used and how fast water is being moved in front of the UV light.
About 2 years ago, all of a sudden, my 125 gallon reef experienced a bout of hair algae. The tank had been running for 3 years prior to that; and the rock/sand came from another tank I had set up before that. So the rock and sand was certainly mature. To this day I am still not sure what was the cause as everything tested out about right. I tried the normal things like pulling the hair algae from the rocks, running phosphate sponges (even though phosphate was not all that high), tried emerald crabs, lettuce nudibranchs, weekly water changes etc. to no avail.
A fellow reefer suggested, since nothing else was working, that I try a UV sterilizer. Within about 2 weeks of using the UV sterilizer the hair algae problem was under control. From there weekly water changes and on/off running of the UV sterilizer erradicated the problem.
That was 3 years ago. To this day I run the UV sterilizer for a total of 48 hours per month and I've never had any issues with hair algae, bubble algae or the like again. Corals are growing well, tons of pods (in tank and refugium sump) and caulerpa seems to experience no ill effects to running the UV sterilizer.
So I guess I am an advocate of running one off and on as preventative measure.