I've had a 42mm Apple Watch Sport for about a week now and I've seen a lot of talk about battery life. I'll admit the first couple of days I had it I was really worried the battery was not going to be enough and I was going to grow tired of worrying it. After a week I don't have that same concern. I now think the battery drain of the first few days was a combination of me playing with the watch constantly and giving demos to everyone I met. That died off after a few days and my usage moved to what is probably more "normal". I wear the watch all day and get notifications on it, but I'm not constantly playing with it now trying out everything. I also typically log a one hour workout either trail running or road biking. All of the workouts involve using the GPS and constant heart rate monitoring.
Side Note: I find that I hardly take my phone out now during the day unless I'm away from my desk and need to write something longer than a simple dictation through the phone.
I left work early because I wanted to ride up the canyon before it got dark. I had plenty of battery left on my iPhone 6+ and the watch for the ride.
My boys and I were also going to the midnight showing of the new Avenger's movie (it is terrible btw) so I knew it was going to be a longer day than normal. I thought about charging it up between my ride and going to the movie but decided I'd really put the watch to the test and not give it any additional juice. Right before I went to bed at 3:30am I took a screen shot of the battery life and it was at 14%.
After 21 hours of continual use with a 1 hour bike ride using GPS and the heart rate monitor, the watch was still at 14%. Not bad. Keep in mind, I don't get a lot of notifications or interaction after 10pm so the last 5 1/2 hours didn't need a lot of power. So far I'm satisfied the battery is sufficient.
Bonus: I discovered tonight while out on a walk that if you double tap the crown, it will switch between the last two apps you have run. I often want to jump between the music and the workout app, now I have a way to do that!