I've personally used them, but we always stay onsite and we've always gotten the free ones. One year we did buy the Haunted Mansion one for our daughter as an end-of-the-school-year gift, as we were about to head out on a trip and she loves anything Haunted Mansion.
I wasn't sure how I'd feel about them, but I very much enjoy using the bands. Convenience is the biggest factor, I never have to pull a card out to use fast passes, gain park entry, make purchases, etc. Also, I feel it makes it easier when traveling with kids. Our kids do not mind wearing them, so I'm not trying to keep track of cards for them also.
Finally, most, if not all, of the pools at the Disney hotels are gated and require a room key to enter the pool area. It's so much easier to just have a magic band on to enter the pool area than have to bring a room card to the pool and then try to keep track of it while swimming.