Dave Mark did some testing to see what his battery life was like while using iOS 9 beta 3 and latest beta of watchOS 2.0. His results were quite impressive and fall in line with my experience on the iOS 9 public beta. I’ve only been using it for a couple of days, but I haven’t seen any signs of poor battery life — I’ve been getting the same 9-12 hours per charge as I did when I had iOS 8 installed.