Austin Air makes really great air cleaners. I used one for about 5 years but didn't want to pay for the replacement filter so I stopped using it. It's not the quietest thing, but to really clean the air, you need to move it. These days, I just try to keep the windows open and get as much fresh air as possible.

I had that Sharper Image one but hated it and ended up returning it. I have one from Honeywell and it seems to help my allergies. I bought it because it was one of the "cheaper" good performing models in an article I read.

Not sure if you are looking for a portable one or whole house air purifier. I have an April Aire whole house purifier that is connected next to your heater/air conditioner so that whenever you use the a/c, heater or house fan, it will purify the home. Works very well and your home will have less dust overall.