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.