I had one on my system for a bit until the pump stopped working and fried the UV system since no flow was going through it.
I think they are okay in my opinion. I think the saying goes, I would rather kill some of the good things in the water column that to keep all of the real nasty things that could be there. Great to keep fish healthy. I don't think it really hurts the coral that much either. If you are afraid it will kill off the plankton, well then be assured that if you feed your fish, you will still be feeding your corals at the same time. And there are other means of feeding your corals too.
So bottom line, I don't think they are a bad thing.
yes Don I actually have 3 on my tank. My reasoning is pretty much the same as what CHuck said. natural food sources are really poor to be honest and and our tanks dont produce much in the way of plankton either.
I have to kinda agree with the previous two post. I just recently started running one and now that I have one will use it on a regular basis until I can find a good argument not to. The way I see it, an ounce of prevention is worth a pound of cure. A lot less headaches too. JMHO...