I really love the GWF! I've had it a couple of months now, and wear it almost constantly.
My sense is still that the GWF that it is more accurate than the Polar for lower HR activities - the kind that put you in the 3-4 Met or below range. It's easier to use for those, too, since you can just slip it on/off, and I love that it keeps me honest about how active I am during the day. Turns out that some days I was barely moving, even though I felt exhausted. I have varied on a workweek days from 1400 to 2300 calories burned, which is quite a difference! Polar is supposed to be accurate down to a HR of 80, and sitting at the computer I drop well below that
The Polar 'seems' more accurate for interval work - but whether or not it is, I like to see the HR change when I'm doing those workouts and the cumulative MET changing doesn't incentivize me as much.
The GWF doesn't work well at all for horseback riding (counts my horses steps and energy) or for stationary bike riding; but I take it off at those times and use the online function at the GWF site to enter in my best guess or my Polar 55 reading, whichever seems more appropriate.
Usually my GWF reads higher than my Polar, until I've been working out for awhile in a relatively steady state, and then they merge (once I subtract out resting metabolic rate).
My greatest successes with GWF so far has been to significantly improve my sleep patterns
