I've been using them for years. I had one on my first Raleigh 3 speed way back when, I use one today on my commuter in the winter and switch it to my touring bike in the summer. By doing that I have one bike with a totally reliable lighting system available all the time. I prefer them over the idea of a dyno hub for a couple of reasons. They are cheaper, lighter and don't create any resistance unless they are being used. I've only had a couple of problems with them slipping in the rain, but the mount had just come out of adjustment. I just bought a new B&M this year and the have wire brush roller I intend to use this winter. That should eliminate any possibility of slipping, even in Michigan slop.
Marc