I’m on the Polar Vantage M (sort of the poor man’s version ! The GritX is sort of the ‘robust’ version of the Vantage V, which in turn is the higher spec version vs M.)
I’m using it for everything – triathlon (original reason) and lots of swimming inc Open Water as well as biking and the occasional run.
3 years of daily use and still going strong. I did a little check of the battery just yesterday- charged, turned an outdoor activity with GPS on and just kept it on all day. After 12hrs the battery was on 51%. So that’s still holding up OK.
Optical HR is always slow to react vs a chest strap on any device and make, so I use a chest strap when doing sports. That helps extent the battery life too.
I really like the look of the GritX, but whilst this one is still going fine I can’t justify buying something else.
My previous Polar HRM lasted donkeys 20+ years ago, and took some hellish abuse in all sorts of conditions.
My subsequent Garmin watches … hit and miss with reliability.
Polar Flow as an App is light years ahead of Garmin Connect too.