In defense of the Dyson DC23 Turbinehead...
I'll go ahead and throw my experience with the DC23 Turbinehead out there - I love mine and find that the turbine head works well with the carpet I have. I'm very happy with its performance. Now, I know everyone has different carpet and vacs perform differently on different carpet media, so I totally respect the bad experiences out there.
Luckily, I am moving to a house with fir flooring, so the DC23 will be in it's element using straight suction and a hard floor tool.
I've never used the motorhead version. I can see, though, how attachments would be very hard to use if you can't take off the extension tube.
As for poorly designed vacs, I'd say any bagless that depends on a pleated filter as the first filtering stage (directly inside the bin) falls into that category. Those filters clog after every use and need to be cleaned off each time - or else no suction! And those filters need to be replaced too often! That's one thing the Dyson tackles really well. At least that's my experience with the DC23's Root 3 cyclone. Good separation and filtration, so very low maintenance in that regard.
Also, any vac that has a cord less than 35 feet does not work for me. OK, I speak as a building manager and it's a pain to deal with extra extension cords in the hallways. I actually love 40 and 50 foot cords. I'd say any vac with a 25 foot cord is poorly thought out.
By the way, I've heard that the turbinehead for the DC23 is prone to failure. I've never had an issue with mine. I'm curious as to what the failure mechanism is on those. One member suggested the brushes would over-rev if the head were not in contact with carpet, and possibly get damaged. Is that how the turbine head fails?
And I guess people don't like the performance of the small hand held turbine attachment for Dyson?