(My standard disclaimer - Don't take any one person's advice as gospel, including my own!)
In my humble opinion, UV sterilizers are, at BEST, useless, for MOST setups...
Now, let me qualify that a bit. If you're running a fish only, or fish only with live rock system, then they can't do much harm, but I really doubt their effectiveness. (Well, for the stuff that goes through them, there's no argument, they're effective. The question becomes, what goes through them that you want to kill?)
For a reef setup, we generally want to get as much life as we can to increase the diversity of the system. And all that diversity will be destroyed when it gets sent through a sterilizer. Yes, they nuke the "bad" stuff in the water, but they nuke the "good" (Or even ambivalent) critters just as readily.
If you've got a particular pest that you know about and want to try to control, then I can see them being useful, but for just general use, I personally recommend against their use...
