Yea, that price for a UV is just out of control. 50 bucks for 24 watts! Wow! I have a UV (tetra UV-5) which is small (only 5 watts) on my 30 gallon, it is a more traditional in-line unit.
I think there are a couple schools of thought on leaving them on, tangyman mentioned one of them. The other is that leaving it on will weaken the immune system of you fish by taking out the majority of pathogens (so if something strong came along or your UV broke your fish would be goners).
I myself only use it when I have an algae bloom or something, but that is mainly because I try and keep my aquarium's power consumption at a minimum. The only annoying thing about that is that it does cut down on the flow of my canister filter.
I picked up mine on ebay for $40 after shipping.
But that one looks good... UV is UV, if it uses a real UV bulb and it passes water through it, its going to work. Not much to them.