I bought this to use to with my Garmin Forerunner 305 and now it's fitted to the bike it works great and I find it really useful. I did, however, find it a bit messy to fit. It comprises of three parts - the sensor, the pedal magnet and the spoke magnet. At first I thought that I was missing one part because the two magnets stuck to each other. On reflection, I suppose it's pretty obvious that this would happen but at the time it took me a little time to realise this.
The included instructions are not very clear but I did find a video showing how to attach it to the bike. Just search youtube for "Garmin Edge 800 Cadence sensor install."