I use the Fenix mainly for monitoring exercise, especially on the bike. And, of course the clock.
The Garmin Connect app is great for keeping track of rides, health, and sleep patterns, etc.
I chose the Fenix over an iWatch because of it being a Garmiin product and I use a Garmin 520 bike computer as well, so, I'm familiar with their processes and it synching to the Garmin Connect app on my phone and OSx Mac.
Also, love the more rugged look to the watch compared to the sleek iWatch. Cost was pretty excessive compared to an iWatch that you can make calls from and text message. I do like that the Fenix connects to my iPhone and receives notifications from email, text, calls, etc. Maps are tiny and pretty much unreadable if lost, but, not any different than the smaller Garmin computers like the 520.
What I wanted basic watch features (time, calendar, etc), heart rate monitor, Bluetooth connectivity (with iPhone for messages, calls, etc), activities tracking, altimeter, customizability, ruggedness (at least for swimming), size on wrist (not too bulky, not too small), weight (not too heavy), long lasting battery charge.
Fnix 5 has these features and more. 3 sizes to choose from.
Challenges (negatives ?) - Price point is a challenge, so you will have to make that leap.
Looked at Fitbit, iWatch - to small and chinsy. I'm not a huge guy, but the sizes of both of those are really for a girls wrist.
I was not certain this watch would live up to the price tag, which is steep. After wearing it for almost 2 months it certainly does. I love this watch. Running, cycling, walking, golf it nicely delivers in all areas of fitness tracking. It's easy to use and appears to be very accurate. I most like the continual heart monitoring. On the Garmin app I can review my heart rate throughout the day and especially resting heart rate at night. After an especially intense week on the bike I noted my HR was relatively high and I modified workouts and rested. Transfers seamlessly to the Garmin app and my desktop. I love the customizable watch faces. Fun!
Highly recommend. And I have been a long time customer of Bike Tires D. and they are the best. Nothing but good experiences with them.