I purchased two of these an the advice of a friend.  I have them in bedrooms.  There is a small amount of dust on the filter thing.  However, i did not notice ANY difference in my allergies, congestion or dust.  Neither did my son.

I purchased these on E-BAY, but they were in original packaging.

So my advice is if you want to give these a try, buy them directly from a store, then you can return them if not happy.