We have a Filter Queen Defender Air Purifier. It was very pricey (DH bought it off of tv for 1000+ over a year ago). I can't say it was worth the price, because I have never used another.
It does work, though. We had our basement flood and mildew formed under the carpets last fall. I plugged the filter queen in downstairs for 2 weeks, and the smell was almost gone. It helps with my allergies as well as odors left by the dog. It has pricey up keep, about 100 a year, but by keeping it up you automatically renew your warrenty with the company. It is almost silent, but can be great "white noise" when on high.

Here's a
review from epinions.