Thats a tough one. a windtunnel with a fresh belt is probably on par with a concept, but a concept doesn't need a fresh belt to beat a windtunnel. I do like that the windtunnel has a hose, but its short and springy and nearly useless at that. I dont like the snap-on pull-off bag door either. The windtunnel is also a machine I get annoyed with hearing rather quickly, but I would never tire of a concept.
The dirt sensor is a little eye that senses when something, small or large, is picked up; its like a motion sensor. but obviously with the eye being in the dirt path, it is subject to getting caked with dirt/dust and yherefore being irratic in their reading. Personally, there isnt that much dirt in anyones carpet that a windtunnel couldnt pick up in a couple passes, yet the sensors always claim otherwise...