I have one of the Honeywell HEPA. It is loud, so we don't use it much. I'm wondering how much electricity it uses.
My wife bought one of the Sharper image one a couple of month ago - due to our friend's recommendation. It is much quieter, but the Ozone smell kind of bother me. It seems to be working, since the amount of dust on my desk is less than before. When you turn it on, you can feel air moving from the front of the purifier. I'm not sure how they do it.
I also see some of the knock-off brand, but have not use it.