Works great for occasional use. I had so much green algae in a 29gal tank (in the water, not on surfaces) that visibility was down to about an inch. That cheap little sterilizer (9 watt) turned the water crystal-clear in about a week. The algae has not returned, by the way, though diatoms are a perpetual problem.
The built-in pump is kind of noisy, though. It's not too loud, but the noise is unpleasant (it has a sort of grinding quality). I don't use it on a permanent basis.