In my very limited experience it's a neat idea, very smooth and silent, but with very noticeable resistance compared to a chain. It may have been that the belt on the one example I tried was too tight - it was very tight, but I was told that was normal - but if the level of resistance I noted was typical, then I would limit their application to upright city bikes where the rider just doesn't care. On any sort of performance bike, I would find it (spin the wheel by hand and watch it grind to a stop in a couple of revolutions) unacceptable.