They are the textbook definition of cheap plastic junk that will last to the warranty period then be a game of roulette!
I have a shark, the Rotator model and it isn't too bad. I'll give it that it does do a good job. But it's plastic as hell, really flimsy, bloody loud and the filter lets everything through. Need to refurb and sell it really and get it gone.
I had another one a while back which had burnt it's motor out. I hit it with my car in the end because that's how I felt!
Shark are just very good at advertising and making things look nice! They do work well, look at the videos by IBAISAIC for the proof,but he isn't the most reliable benchmark as his don't get used that much, and when they are, he shows the state of the thing afterwards! I've used the Shark that HooverLux had and it was very good. Stuck to the floor, groomed really well, the soft roller on the front is good, but was already filthy and Hooverlux has clean carpets, and the folding handle joint was oh so creaky and wobbly, he said it was out of the box. AND VERY LOUD! Makes a Kirby seem quiet as the kirby has a low pitched roar, this thing just screams it's little plastic head off, same with mine.
Sadly Dyson aren't far behind. I've had to wash the filter on my V11 today 3 days after it turned up because the sodding thing still turns on with the bin open. Average Joe does that more than a handful of times and the thing is toast in short order!
All you can do is try one and see, they have a money back guarantee as they all do now (I get 35 days with Dyson) so you won't loose anything if you get it used alot asap to try!