I have used the polar series of watches both for cycling and running. The forerunner is a much better bit of kit as it tracks your real ground position. This gives distance, pace, timing and other data to an accuracy of more than 99%. I really like the autolap feature, the courses and the virtual partner settings. Unlike the Polar, the speed and cadance sensors work well on the bike (an optional accessory).
With the ability to analyse the data using their free "Training Center" it is fairly easy to review your progress and plan the future. This software does a decent job of data anlysis but if you really want more there is the option to do some further data massaging in Excel by importing the history file there.
For 5 stars it needs these changes:
Longer battery life (currently about 10 hours continuous use)
better water resistance (I swim and am too afraid to use it)