I havent found that to be the case
Hi,
My mum has had a DC03 upright for 6 years now, and never any issue with dust clogging or belts. The only issue which has occured, is the need to occaisionally remove the brush bar and remove any dust from inside it. But other than that, and washing the filter every 6 months or so, there has never been a problem.
We've got a DC08 pull around, Its performance is great, I even had it at work cleaning up plaster and masonary dust. Nothing ever got through the filter. However I find the hose mount is a little awkward to tug around.
Did the appliance place you visted actually sell the machines? They may be plastic, but they take a beating.