It was almost too good to be true.  I was using [my Polar M400](http://blog.chukhang.com/2016/01/review-polar-m400-and-flow-as-activity.html) every day and it was doing almost everything I had wanted it to do.
However, the convenient micro-USB charger port turned out also to be its Achilles's heel.  After a few months, the port started getting finnicky and the watch refused to charge without some "jiggling" of the USB cable.  Over time it got worse to the point where I could not reliably charge the watch at all.
I sent it back to Polar for service.  It was hard to let go of a watch that I'd been wearing almost 24h per day for nearly a year.  Fortunately, Polar service turned it around reasonably quickly: it took 8 days from when I shipped it (USPS Priority Service) to when I received it again.
The repair summary was `Oxidation in Connector`.  After a bit of Googling, it appears that this is a common problem with this model of watch.  The problem affects the early models (including mine) which had a rubber cover for the charge port.  Sweat would get under the cover and the cover prevents it from fully drying.  Over time, the sweat corrodes the port.
The repair replaced the port and removed the rubber cover.  [Supposedly the port is already waterproof and the newer versions do not have this rubber cover.](http://support.polar.com/us-en/support/maintenance_of_your_m400)  In fact, there are some people who recommend removing the rubber cover if your version has it.
So overall, it wasn't a bad experience.  It was annoying to be without the watch for about a week.  It was also annoying to have to pay $6.80 USPS shipping to send it back.  However, at least they not only repaired the watch but they also addressed the root cause of the problem (the rubber cover).