I hope this thread is useful.
I've had my Fenix 5X for just over 3 weeks now. I had a Fenix 3 and F3 HR for two years before this. From my experience pre-ordering the F3, I'm amazed how stable the F5X is at this early point.
Obviously the maps are the main reason I upgraded, but I feel like there are a thousand subtle improvements, mostly ones I wasn't expecting. The watch feels more refined and polished in day-to-day use. Here are some of the things I like about the F5X after using an F3 HR:
(1) Maps and map-related features, this is huge.
(2) The watch is faster.
(3) Display is better and easier to read when exercising.
(4) Power gauge for cycling, like the HR gauge for running.
(5) Countdown timer shows the time of day. I use this a lot for cooking and washing my sweaty workout clothes.
(6) Much quicker to calibrate a power meter before riding. Instead of having to go deep into the menus, it's the first item now!
(7) New fitness features are nice, I wish I had them for XC skiing all winter.
(8) The watch is a lot more comfortable on the wrist than the F3 HR because of the sensor bump. Vibrations are easier to notice as well.
(9) Resting heart rate is a lot closer to reality.
(10) You can re-order data pages for activities! I've wanted this on my F3 a few times.
I'm sure there's a lot more that isn't coming to mind.
Edit to add:
(11) You can add the altimeter/chart as a data page in an activity. Compass too!
(12) App menu is much better. Short list of favorites, then everything else relevant, and if you do an occasional thing, the watch still remembers the last favorite you used.
(13) The quick start menu is nice. I like that it shows you the time on the timer/stopwatch without having to go into these.