It’s very early in the product life cycle, but anecdotally, I have found the battery life to be very good. I plugged my new iPhone 6 Plus into my computer to do the sync/setup when I first got it. It had about 80% battery when I started, and was up to about 93% when I finished. I played with it all afternoon and evening, and I was still at about 48% when I put it on the charger at bed time.
That is on wifi mostly, but I was out of the house for a bit, and was using LTE. With my iPhone 5S, the LTE signal in my neighborhood is on the fringe, so it chewed through that battery pretty quickly. The 6 Plus did not seem to be bothered by the weaker LTE signal.