In reply to Renfrew1:
Obviously you will get the whole spectrum of experiences given a large enough sample size but FWIW here's mine.
I upgraded from a Fenix 2 to the Fenix 5 (the middle one, not the S or X) back in August. I chose that one for a number of reasons.
1. It has the longest battery life of the three
2. It's not TOO expensive
3. The screen is higher res than the 5S
So far its been pretty decent with no lost activities or software glitches. I wear it all day long as an activity tracker as well and it's good having a day-long HR analysis (although I concede it's not as accurate as a chest strap) to see if I'm over training.
I used the notifications from my phone but turned them off as it got annoying. It's useful to have WhatsApp/SMS appear on the watch during long runs though.
I thought I would miss not having a WIFI connection but as others have said the activity is normally synced to my phone, uploaded to Garmin connect and synced to Strava before I've even got my cool down finished, and I would want it syncing to the app anyway as connecting to wifi in non-home locations would be a ball ache.
Maps would be cool but I think I'd be nervous of putting all my navigation faith into the watch - the breadcrumb trail feature is good enough to use with a map for new runs.
I like the quick release bracelet too - so I can switch between a skanky running bracelet and a normal everyday one.
The battery is pretty decent - I recently ran the CCC and the watch was down to 50% by the time I hit Champex-Lac at 56km and 11 hours in so a bit less than the predicted 24 hours pro rata. I did have HR running. I topped it up for good measure for about 15 minutes and it finished the race with plenty to spare (in 22 hours).
I tested ultra-trac at home (which is meant to give around 50 hours battery) but wasn't impressed with the accuracy of the recorded GPS. The watch is meant to use the internal accelerometer to fill in the gaps but I didn't see much evidence of that. Just one test so far though.
Its much quicker at picking up a satellite signal than my old Garmin which is great when you're stood around in the freezing cold waiting for it to lock on.
Hope that helps.