I have been using the Garmin Fenix 3 HR for the last three weeks and love it. For anyone who wants to track multiple statistics about their workout, this is a watch that has almost every feature that you can think of and some more. I have used it to track my swimming, running, sleeping and heart rate so far and intend to take it rowing soon. Here are some quick thoughts.
First up, its a big watch and for those who have thin wrists, it can wear a little uncomfortable. You get used to it over time, though I still find it uncomfortable when sleeping.
When using it for running, it picks up the GPS signal fairly quickly and tracks your start location, route covered, speed, distance, elevation and heart rate etc. Based on your run, it is able to tell you your cadence and VO2. I compared the results by using mapmyrun on my iPhone and found the results similar. I was able to track my peak heart rate and the change during the run which was extremely useful.
I used the Garmin while swimming. While the company does not claim to be able to track your heart rate under water, I found the heart rate tracked working fine. It was able to track the distance covered, though it seemed to miss a few laps, speed and average SWOLF.
It obviously tracks your steps, and reminds you from time to time if you have been stationary to MOVE. It syncs with my phone fairly easily and setting it up was easy. To review your activities you can use their web based Garmin Connect where you can track pretty much everything you have done. I particularly found it useful to understand my sleep patterns which showed that on average, 70% of my sleep was deep sleep.
In summary, barring the size, I loved the watch.