Air Purifiers

Looking for an air purifier. Saw one at Target that never needs a filter change–you just vaccume it when its dirty. I'd like to buy it so that I don't have to continually buy filters but will this work as well as others? Forgot the name of it but if anyone has this type can you tell me about your experience with it?

Also they all advertise that they have "quiet operation" but which ones are really quiet?

Can anyone recommend a good one?

I have asthma and allergies and had an ionic breeze unit that died - I am looking for an air purifier for a medium sized room - cost is not as important as quality and quiet operation - thanks!