I had my Fenix 3 for so many years, and I loved that watch. In fact, there was nothing wrong with it when I "retired" it and gave it to my husband, I just wanted a heart rate monitor! The extra battery life with the 6S was only a plus. So far I have been just as happy with this watch as the first one. From swims to runs to backcountry skis, I use it everywhere. Even the occasional surf!
It being smaller than my old watch was a plus as well. When I bought this watch, there was the option to go with a model that was the same size as my 3, and had longer battery life, but I don't feel like I am missing anything by having the smaller one with less battery life.
PROS:
Can track most any adventure, and is a great companion for any workout. I named my watch "Joy", and she really accompanies me while I find joy in my physical body.
Smaller profile - I don't hit it on things like I did with my 3!
Intuitive user experience asides from downloading maps.
Can choose to not have Bluetooth on - I hate getting text notifications on my watch so this is a plus!
CONS:
The HR monitor piece on the underside of the watch is fragile. The first one I got I accidentally broke within a week because I dropped it. The rest is sturdy!
Like most other GPS watches, loading maps on to it isn't very easy or intuitive.