I had one for a while, when I had short hair. It was a huge hassle and expensive to get haircuts often enough to keep it looking good, so I bought a Flowbee.
At the time, my daughter was around 18 months old, and freaked out by vacuums, so I'd have to lock myself in the bathroom and make sure she was entertained by my husband (which added a layer of complexity that most people won't have to deal with).
It did a pretty good job. I'm sure I wouldn't have won any beauty awards, but it was nice to be able to get a haircut on my own schedule. I found it to be a little hard to learn to work with. I had the vacuum that came with it, and it worked fine for me.
I never had to replace any parts--just used the ones that came with the set. When I decided to grow my hair out long again, I sold it on ebay and recouped most of my original cost.