As a general rule, I don't think they change the water clarity over the long run. They are great for getting rid of green water/diatoms. That condition is generally temporary and would eventually clear on it's own. They will also kill any living organisms present in the water column, thus helping to prevent white spot. I don't think they have any effect on algae at all other than the green water. They will not prevent other types of algae from growing on plants, decorations and other surfaces in the tank. If it doesn't pass through the sterilizer, it won't be affceted by it.
Having said that, my answer is yes, depending on what your situation is. If you have a green water problem then they are absolutely worth the money. If you are fighting white spot every few months, then get one (or figure out what is causing it). I have one that I used to rid 2 tanks of green water about 2 years ago I haven't used it since then but to me, it was well worth it to get my tanks crystal clear again without chemicals.
