if you update IOS 8 onto a 4s you brick the phone. My wife's 5 on iOS 8 is just fine
iOS 8 is fine on my iPhone 5 but, but it seems a little bit slower, especially Safari. I never had any Safari reloading tab on my iPhone 5 on iOS 7, but now opening 3 tabs or even switch from normal to private and back to normal will make all my tabs reload.