The GPS reception always seemed to be patchy – it’s absolutely bob-on while on the bike mount, but when running it would record weird paces, the position would flick from one side of the road to the other and it regularly took massive short-cuts that I didn’t.
I think the filtering algorithm for recording data is tuned for bikes, and swinging it back and force doesn’t help.
The Forerunner also gives you stride and cadence metrics, pretty good reception, and is barely noticeable. A really useful feature is that it can be set to vibrate when it needs your attention (lap / pace / heart rate etc) which makes for a far more pleasant run than with something bleeping at you.