I think most people, or maybe this is just why I am kinda frustrated, is because iOS 9 was touted for its better battery and optimization (with much less features than other updates). The fact isn't how much worse iOS 9 is compared to iOS 8's or 7's release, but the promises Apple made for this release. Optimized? I don't see it with the amount of lags and stutter that I am getting on a 6... Battery life? My phone is dropping percentages faster than it did on iOS 8.4.1. I can only squeeze 5 hours of usage time (and we all know that isn't screen on time...) on iOS 9, but when I was on iOS 8, I could get 7. That's a huge difference.
Bottom line for me, is that Apple promised X Y Z, and did not fulfil X Y Z, and therefore, that is why I am more frustrated than usual. If they didn't, I would blame only myself for updating/upgrading because that is expected. Although it is still expected that these should occur, do not promote the update as something it is not and then being unable to fulfil those promises.
I know Apple did not promise SPECIFICALLY the things we are dealing with at the moment, but you have to admit that what iOS 9 essentially was promoted as was not really what they shipped out to iPhones 4s-6 NOR for the new iPhone 6S, considering the problems they're encountering as well.
EDIT: I am also not a power user. I don't have many apps downloaded. I use it to check Twitter some times, Instagram, Facebook with Paper, and that's my main apps. The next ones are Messages and Music and Settings. Literally, Messages or Home and Lock Screen are at the top. It said 3+ hours?...Well, not only did I NOT get the SAME amount of battery out of the same usage, I got LESS. That is why it was frustrating and I am upset with Apple.