Personally, I nave used UV in the past. I am not a big fan of it, unless you are using it for a very specific problem.
UV is basically going to kill bacteria, algae spores, protozoans, and the like that pass through the UV unit, provided the amount of UV they are exposed to is enough to do the job of killing. If what your trying to kill doesn't pass through the UV unit, it will not kill it.
Generally, it takes more to kill a protozoan compared to algae spores, but there are exceptions.
Unless you use massive amounts of UV it will not kill the most common SW parasite diseases SW ich and SW velvet.
UV can help keep water clear by killing bacteria and algae actually in the water column. It will not kill things like hair algae growing on your live rock. The downside to this is that many corals and other filter feeders consider this bacteria and algae food.
Generally you do not need to add anything extra because you are using UV. While we are on the subject of additives, I recommend that except for the calcium and alkalinity additives you stay away from them. Rely on partial water changes to keep the nutrients and trace elements up.
I recommend that you save your money and don't use UV, unless you have some very specific condition where UV will help.