I was a long time Garmin user but not anymore as I wanted BT Smart compatibility. So after some other watches I bought Polar M430 and hating it changing it to Vantage M. Some points from my initial months of experience, hope it's useful.
1. Wrist based HR is working properly only when it's neither cold nor very humid (or both). OR, when pushing hard. This could be due to my individual characteristics, but some support mails from Polar confirm it's a general issue. So I need a chest strap for any runs where I'm going above easy pace effort.
2. The black background is OK in daylight only. At night it's pretty much useless if you don't either push light button (good backlight but annoying in the middle of a hard interval) or raise the arm - but then the backlight is very weak and it takes 2 seconds for the watch to react. So no good functionality here.
Polar should release a white background setting immediately.
And/or release a "permanent backlight" function (as has the cheaper M430).
3. The watch lacks some basic functionality for advanced sessions such as the possibility to move to next session phase manually. Now if you set up e.g. 30 mins warm up for transport and warm up drills but end up ready after 20, you need to stand still waiting for 10 minutes. This is REALLY annoying as it's often not known how long time it will take to get to the track etc. (Again and stragely, the cheap M430 has this ability.)
4. The GPS is really weak, as already mentioned above in thread. I often get a 3% deviation (less). Polar confirms the GPS is significantly weaker than the Vantage V. It underreports even when I use it for bike sessions where - during my commute - I bike on really straight good roads for a long while. Strange.
5. The indoor distance measurement is a disaster. For me it underreports with 15% distance.
Other than that, the Vantage M looks great, feels really good to wear (very nice wrist band) and syncs quicker and more stable than most. Battery life is also good.
I'm overall OK with the Vantage especially as most of my concerns can be fixed with software releases (maybe even some GPS correction). But Polar needs to get their act together before long if I'm not ending up dumping this as well on the second hand market, moving back to Garmin.