WARNING:
Consumers should never forget that Apple is a hardware company. Software resources within the company support ONLY THEIR NEW hardware as the highest priority, then maybe they fix other issues later. Support of older devices is WAY DOWN the priority list. Basically if you have an older device, DO NOT EVEN CONSIDER upgrading to a new iOS Major release until X.N, where N is the number of hardware "things" released off of the iOS code base at/after the major (X) release.
So for example in the case of iOS 9: X=9 (the major release), and 5 iOS related devices (6s, iPad Pro, TV, SE, 9.7" Pro) so X.N = 9.5
Maybe we can consider the 9.7" Pro minimum churn and N = 4 (and since it was released with the SE), but the poor quality of 9.0-3 so far still makes the entire major version 9 suspect. Regardless, don't upgrade older devices at least until the first non-hardware-release-tied update of 9.X.
Essentially:
iOS 9.0 should have been called iOS for iPhone 6s and 6s Plus.
iOS 9.1 should have been called iOS for iPad Pro.
....etc....
And, in the mix they shipped Apple TV, which is basically iOS and "churns" the iOS code base.
I've been an active iOS developer selling apps since iOS 2. iOS 9 takes the cake for "Worst iOS Quality Ever".
My everyday devices are still on 8.4.1, and I'll probably skip 9.x altogether.
Apple is starting to slide back to the ancient era mindset (30+ years ago) when an Operating System was created solely for a piece of hardware (even general purpose computers) - Think TRS 80.
You've been warned. Yes, Apple is the new Microsoft, for this and several other reasons.....