I have an iPhone 5 and whatever they did in 8.4 is causing it to dramatically eat battery life. I don't use and haven't used Apple Music so it's not that which is doing it, which I've heard people complain about using battery life. Thursday and Friday I hit less than 20% easily without using it much and that is a rarity that only usually happens when I am using it for most of the day. I'm really disappointed with this update. I was happy with the initial iOS 8.0-8.3 updates as they didn't seem to affect performance or battery life anywhere near as bad as the previous updates that were 2 years after the models came out. I was holding off buying an iPhone 6 to wait for the iPhone 6s.
If you can, I would suggest updating to 8.3 and leaving it there unless they put out an update to fix the battery drain issue.