I have a Fenix 5.
What I like:
I agree with an earlier comment that the Fenix line is bullet proof. I’ve fallen on it, dropped it, whacked it on stuff in the shop (crankarms and chainrings being the common culprit). It’s a little bulkier than I like in a watch, but again, durability is awesome. Feature-wise, it’s a great stunt double for a smartwatch. Does a decent job tracking heart rate on a bumpy ride, lots of apps/watch faces to choose from, good/great battery life. Pairs with every phone I’ve had in the last couple years, easy to pair with your sensor accessories (you’ll need speed/cadence for better distance tracking). Very easy to set up and use the few dozen sports widgets that come with, in addition to the tri sprints I’ve used it for DH/XC skiing.
Things I don’t:
Price, even a for a refurb one, is pretty high (I got mine brand new). This isn’t the one that I can load music to, sadly. That was released about six months after I got mine (definitely wasn’t happy).
The bulk is the other thing I don’t like. It’s big (it’s not as big as the 5X, but still) and weighs just enough that it is really hard to get the band tight enough on my wrist so it doesn’t bounce around on the gnarlier trails…and go too tight and it cuts of circulation. I’ve kinda stopped wearing it on my mountain rides and wear it for a gravel one instead.