In reply to Richard Carter: I have a 620, and have also tried Suunto (I have an Ambit2 Sapphire that I must get round to selling).
The Garmin's great as I can wear it as an everyday watch without looking like I've got a dinner plate on my arm. Garmin's a better runners' watch than the Ambit, as the Ambit is a bit clunky, partly because of all the extra functionality it has (compass, altimeter, navigation etc). Suunto also doesn't work directly with Strava.
+1 for DCRainmaker's site - really detailed reviews from a bloke who runs and bikes a lot - so knows how runners and cyclists use stuff. The Polar is too big for me, as I like to wear my watch every day - and I'm not sure the lack of ANT+ support is a great move, as lots of us have all sorts of ANT+ devices already.