UV sterilisation is a good way to kill bugs in water, they often use it in poverty stricken countries who can't afford more expensive sanitation methods. It would be great for a tank which was prone to bacterial blooms or algae problems, but it is not intended as a filter substitute.
It would kill nasties like bacteria yeast and algae, but would not offer perfect water parameters like a regular filter. It could not simply "sterilise" ammonia being a nitrogen/hydrogen compound, you would need a proper biological filter for water quality.
I have never used one but i think it is designed to be used in conjunction with a filter to offer another level of protection against bacterial/fungal infections.
If you were considering it i would look into wether it affects the water temperature, considering it uses a form of light and it has mechanical parts in the water.
