One of the cool things it does is measure (or estimate) your VO^2max. Below you can see the trend since I got the watch in late November. I hadn't been running a ton before then so you can clearly see a trend of increased fitness.
Essentially it measures your heart rate and pace, and uses a formula to calculate your estimated VO2max. You can see the formula in action on the two points I highlighted. Point (1) I visited Mesa, Arizona and ran in Mesa (1,243ft ele) one day and the next day in high elevation Flagstaff, AZ (6,910ft ele). It makes sense that I would have a higher pace / lower heart rate in Mesa than in Flag, especially considering my buddy and I summited Elden lookout (~9200ft) on the run in Flagstaff. Also, on point (2), I was traveling to central California where it is around 375ft elevation. Now I'm back in Central Oregon so I'm sort of back to my current baseline of 57.
Hopefully I'll update this again later in the year, after a few more key workouts and races.