My 16 year old son bought me a cordless Dyson for Christmas one year. He was only working part time at the moment and it was a lot of money for him to spend. I am so disappointed with it. The battery gave up after only three weeks. However, Dyson replaced it with no questions asked.
The head of the vacuum is not very big and it does not pick up a lot of stuff most vacuum cleaners usually handle with ease. The roller brush in the head of the vacuum gets jammed up with my daughter's hair so I am constantly taking the roller out and have to cut the hair out of the brush.
It's ok for picking up dust but apart from that, I would steer clear of them. In my opinion, they are not worth the big price tag.