I use one and swear by it. Ed's right of course it won't stop algae other than green water algae, which it does very efficiently, but it will help to keep your fish disease free. Especially when combined with a healthy and varied diet and effective quarenteening of new arrivals. In the last few years I haven't needed to use a single drop of any disease remedy in my tanks so maybe the ammount I save on the cost of medicines may go some way to paying the the running cost of the UV filter.
Regards, Chris.