I guess if you after cadence data for your rides, and you already own a Garmin your choice is limited! I am primarily a runner, I have a Garmin Forerunner 410 GPS watch. I own a Trek DS 8.3 hybrid bike which I have been using quite a bit recently so I thought I would add a speed/cadence sensor to give me a bit more data from my rides. It arrived nice and quickly and has a reasonable set of instructions for fitting it. I found the required magnet positioning of <5mm on the crank sensor was just not going to be achievable on my bike due to the more 'mountain bike' style frame, the crank is quite a way from the rear frame stay. The spoke sensor was fine and lined up ok. I did some research, seems its a fairly common thing with this sensor unit (I am sure it was designed for a road bike frame). I ended up purchasing a neodymium rod magnet 10mm dia x 15mm long (just a strong magnet really) for about £3.50 on eBay (you can get them in just about any size easily). I then ditched the Garmin supplied crank magnet and stuck the new rod magnet to the end of the peddle spindle (it sticks on its own, magnetically, as the spindle is steel). It stays in place ok, been for lots of rides. This allowed me to line up the cadence sensor unit on the frame with around an 8mm gap to the new magnet (being more powerful than the original it trips the sensor ok). The spoke magnet was still fine, so I now had a fully working speed/cadence unit on my hybrid. Enabled pairing for it on my Forerunner watch and off I went. After a mile or so it had automatically calibrated the wheel circumference and that was that, been working a treat ever since.