The Dyson fan, while cool, has a much higher pitched noise then other fans. To me that part is a bit of an issue. On top of this, the airflow gets better at a distance from the fan, so you can just place your hand in from and say, ohh yeah power. Not planning to get that any time soon.
Of they made the base column bigger, they could use a larger motor/impeller to drive the air flow and do it quieter.
Can honestly not fault the vacuums aside from some minor things with the first models. They do actually work as advertised(but you better clean them from time to time, still far less cleaning needed then the clones).