Ok, so I really liked the watch interface. It gives you a lot of information. You can swipe across screens to get several pieces of info on your watch screen.
I think what will matter to most people on here, it appears to track your heart rate from your watch. I noticed it on one of the screens when I was done, but then I closed the app. When I looked at my run this morning, it had a graph of my heart rate across the run. How accurate? I don't know, but it's there.
I did a short run; about half an hour/5km, and it used about 6% of the battery.
I was using the free version of the phone app, and not the pro or premium version of Endomondo.