The german magazin "Stiftung Warentest" (a _very_ respected organization) did some extensive vacuum testing a few weeks ago.

You can see the ranking here (unfortunately in German):

http://www.stern.de/wissen/technik/stiftung-warentest-ueber-...

The tested Dyson made last place (from 12 contestants) with mostly average results. It's the most expensive and least effective machine they tested. The biggest disadvantage is the sound level: it was so noisy that it was downgraded.