Just as an update, I did a semi-scientific piece of research yesterday. I emptied our DC-07 completely, then vacuumed our entire house. We have a 4-bedroom, fairly large house. No kids, no pets (apart from our gerbil!). We keep it clean, neat and tidy. Carpets all round.
After the cleaning, there was enough crud in the Dyson to fill a coffee mug, maybe even a little more. For an already-clean house, I'd call that a good effort!
maximus otter