I have one and I find that it doesn't do much for algae. I use it for elimination (or control) of parasites and disease, should there be any outbreaks. More of a preventative it seems.
FWIW, I have a 9w turbo twist running through an Eheim 2026 (~250gph) that has a 300w hydor and the UV hooked up to the outflow. This is on a 90g tank.
I use one for the same reasons listed above... basically prevention. Since I have discus, I wanted to be sure that I had a way to kill bad bacteria and protozoa… and although I have never had green water algae, I know it will definitely take care of that as well.
To add to my previous post, my reason for installing one was parasite control. I had an outbreak of ich, due to not using a quarantine tank, a year ago. I lost most of my tetras to it and a lot of plants after adding salt to treat it. So I decided "an ounce of prevention is worth a pound of cure". I went with a 25 watt for my 55 gallon which should be more than adequate for parasite control in that size tank.
Do you need one? No........Is it nice to have one? Yes!
Water quality is much better, period. As for algae control, it DOES help with that in some instances....I have never seen a bad bout of Green Water with someone that ran a UB filter *wink*
Anyway, I recommnend one if it doesn't break your bank so to speak. I run mine 24/7
I agree, a UV is a worthy investment. On initial setup I always run a UV for several weeks. For one it does prevent and get rid of green water. Green water normally makes other forms of algae worse. My UV is portable between tanks, I run it as needed.