With my S0 and OS3 I was getting better BL than with OS2, and it was quite good easily a full day which would include a 1 hour workout or 30 min walk tracking.. and what read as about 4 hours of use.
With S2 and OS3 I'm getting even better. the battery is lasting me now TWO DAYS with one 1 hour workout in the middle. That's having the watch on 24 hours a day, at night as well and not taking it off to charge till at least going to bed on the night of the 2nd day. For me that's fine amount of BL and not having to hassle with an every day sort of thing. Historically, I would just either take it off at night or take it off while showering and getting ready and that was enough to have it work all day and into the following morning. Now, I just put it on full in the AM and worry about it on the 2nd night.
At the moment, It's sitting at 32%, this is day two mind you and I took it off the charger yesterday AM at 05:45 pst (it's now 12:30 PST). I used it quite a bit yesterday, did one 10 minute call on it to test the speaker (louder! and now really pretty usable), lots of messaging, scribble, weather, and a 45 minute metered walk (with phone in pocket so no onboard GPS used) and USAGE shows 4:25 minutes.
So, I'm pretty satisfied with what they call better battery life.
Oh, and I did a fair amount of indoor and outdoor testing of the new screen brightness (going outdoors, and also shinning my iphone light on the screen to increase it to max and then back down, etc.) I think the screen OFF feature works in about 100ms as it really wants to get off that 1000 lumans mode as quickly as possible.