Polar G1 GPS Speed and Distance Sensor for FT60 / FT80
The unit works well when it comes to measuring the total distance of your run and the average speed. It does not work well when it comes to functioning as a "speedometer" during the run (e.g. showing the current speed), as it is fluctuating too much. It was a bit heavier than I had expected, but the elastic band works well and after a while I did not notice wearing it. My main objection against recommending this unit would be that you can get a combined GPS / heart rate monitor (i.e. eliminating the need for a separate GPS unit) from Garmin for about the same price (e.g. Garmin Forerunner 305 GPS Receiver With Heart Rate Monitor).

I was a bit concerned about this item when I read the problems that some people had experienced using this accessory. Problems with battery voltage and turning off after a short time. I ran a 6 mile run with it and there was no record of the run when I returned. The documentation gave the impression that the GPS unit would be already matched to my RS800cx Polar watch. I went through the matching procedure again and ran 7 miles through Boston Common and along the Charles River (on vacation). This time the the .gpx file opened in Google Earth and showed the whole run accurately covering the path that I had taken. I have used the same AA battery for the 3 runs used to date with no problems.

Although polar has now moved to have the GPS in the wrist unit on some models, I like have it separate because the GPS tends to be a big battery drain. This unit will last an entire day while most of the wrist unit GPS will not.