I have a brand new 6 Plus and the battery life on iOS8 was great. After owning it for 2-3 days I put iOS9.0 on it and the battery life was terrible. I took a friend's advice and downloaded the 9.1 public beta and turned on low power mode. So far so good. Sadly i'm afraid to turn low power mode off, but i'm sure I will eventually.
While I cannot provide detailed insight on non-low power mode on 9.1 I can say that 9.0 was really killing me. I looked at the battery usage and the only thing I could see that really used up battery life was my DirecTV app, but I hadn't even used it that much.
Oh more detail. This is a 128GB 6 Plus and I set it up as new.