We bought one from Hamilton Beach (on sale at Amazon.com with free shipping!) a little over a year ago. Our house is about 100 years old and it had an odd, sour smell in the bedroom. We chalked it up to the age of the house, but when we plugged in our air filter, it said the air was absolutely filthy! I closed the door to the bedroom and let the thing run for a few hours-- the air smelled really fresh when I went in later.
We got to looking around and found an old leak in the attic that was draining behind the bedroom walls. It was full of mold! We fixed the leak etc. but still run the ultraviolet light on the air filter-- from what I've read, it is the only thing that will kill mold spores as the float around and keep them from reproducing.
It has a screen on the front that tells you the quality of the air, and you can set it to run on automatic where it adjusts itself acc. to how much mold, dust, etc. is floating around. I love it, and it makes a big difference!
HTH,
L