I’m using iOS 9 on my iPhone 4S and it seems to work just fine for now mostly.
I did have one app crash with the built in calendar the other day, had to force the phone to shut off. After? No more issues.
It just goes to show you how different phones react to the same operating system. Having said that Apple just released an iOS update patch 9.0.1. Was it just yesterday? I’m sure there will be more of these coming in the next while. Ah, can’t live with them and can’t live without them.