So, because other bagless vacuums don't retain dust well, that suddenly makes it ok for Dyson to start blowing fine dust back out of the machine? I'm not buying that!
It sounds like Alex is disappointed with the actual cyclone set up, but for me, the biggest frustration would be the filter. Even if the cyclone did inevitably let some fine dust through, I would expect this to be captured on the inside of the filter. In my opinion, the biggest flaw is the poor seal between the filter and the cyclone, which is letting very fine dust particles through to accumulate on the outside of the filter housing and blow throw the machine and back into the air in use.
If this is the build up after just 1 week, what would it be like after a month, a year or 5 years? Alex is lucky, knowing what he knows about vacuums, he can intercept this fault before it becomes a problem, but your average user - even those who regularly maintain the filters - aren't going to concern themselves with this and it could become a problem later down the line if dust is regularly passing through the motor. Doesn't exactly make for a spotless home either.