So much dust in my flat! Thinking of getting this air purifier – any feedback?

There is so much dust in our flat that it makes me sneeze. We vacuum every 2-3 days but we do have any books and small items. While I am intent on de-cluttering, I was thinking of getting this air purifier – http://cgi.ebay.co.uk/NEW-PREMIAIR-99-EFFECTIVE-HEPA-AIR-CLEANER-PURIFIER_W0QQitemZ120221274028QQihZ002QQcategoryZ43510QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

What do you think? Are these purifiers effective?