Strengths: The watch is less cumbersome than GPS rivals. The foot pod used to measure distance is pretty accurate.
Weaknesses: Calibration can be a pain and all measurement is based on the calibration - running style / stride difers between short and long distances so must lose accuracy. Battery on foot pod fails at bad times (e.g. on long runs). A replacement foot pod costs £70! Controls are not very intuitive.
Overall: This is a very good watch which does allow you to measure distances run (the main reason you would chose this model). I believe the measurement is fairly accurate but (and this could be unfairly), you always doubt the watches ability to be as good as a GPS.