I found someone who posted this on another forum while doing a search for UV sterilizers..
"While you may get different opinions on whether or not it's needed, UV has no negative effects. Since only water that passes through the unit is treated, there is no impact on the beneficial filtration bacteria. UV does not add or take anything from the water column. The only immediate noticeable effect may be an increase in water clarity.
Contrary to popular belief, UV exposure does not kill the organsims that pass through the unit. It simply uses UV radiation to sterilize single cell organisms and some small parasites so they can no longer reproduce. Hence the name UV sterilizer and not UV killer or UV destroyer (sounds like a marketable name though). Larger organisms like pods and such will pass through unharmed and unaffected by the UV exposure.
I run it on my tank on the input side of my recirc skimmer. There is a theory that UV will affect phosphate bonds and can possibly increase phosphate removal through skimming. Whether or not it works, I don't know. Since I like to run UV anyway on a tank, I figure it can't hurt. I say make the deal if it's that good."