My battery life on my iPhone 6 with iOS 9.0 was as good as with iOS 8.4.1.
My iPad Air 2 appears to be eating the battery much quicker with iOS 9.0.1 but I noticed
though some strange operation of the battery indicator this AM. So I'm thinking the
battery sensor (or what ever determines the battery level) may need to be re-calibrated.
So far my iPhone 6S seems to have very good battery life.