As one data point:
We have have a Dyson model DC-17 (? - It's gray and purple and clear plastic) that we bought about 6 years ago. I think we have had it long enough to give an honest review, though.
On wide expanses of carpet, its cleaning ability is excellent. We have a wiry-haired dog and it gets all the gook out of the floor. Better than any other machine we have owned, which consists of mostly Hoover machines. But its edge-cleaning ability is poor - to clean around the carpet around the baseboards requires deploying the suction wand, which is one of the machine's weakest points. Suction with the wand is good, but the attachments are not that great. And deploying/stowing the wand and attachments is not very good, either.
Cleaning on bare floors is mediocre at best. Chunky stuff like breakfast cereal spills tends to get propelled backwards by the brush but not caught by the vacuum part - frustrating. Poofy stuff like dog hair gets blown around by the motor exhaust and not picked up. Same with small leaves dropped by houseplants - they get kicked around but not sucked up.
The head of the vacuum has very little tolerance for things like bobby pins, paper clips etc. Stringy stuff can cause the brush roller to jam and strip the cogged drive belt. That happened to us once, but a local vacuum shop fixed it under warranty. We haven't had the belt get stripped since, but we are more careful about picking stuff up before we vacuum now - and I think having to do recon ahead of the vacuum cleaner is BS. Whereas when we had the Hoovers, we would vacuum without particularly looking for what might be in the way, even though the Hoovers had lightbulbs that the Dyson lacks.
Using the wand and attachments on upholstery and carpeted stairs is so-so at best.
Dumping the dirt chamber is a little more iffy. Sometimes you have to kind of scrape out the dirt chamber or whack it a bunch of times against the trash bin to get it to empty. Whacking the dirt chamber against the trash can introduces a bunch of dust into the air which kind of defeats the purpose of the hepa filter.
Overall I'd give it a B-, but given the price, it only merits a C-.