I upgraded my iPhone 5 to iOS8 yesterday. Everything looks fine, works fine.
There was just one glitch, automatic brightness is too dark, very hard to see the screen. I turned it off, upped the brightness, then turned off and on, then reset (hold down power and ‘home’ button at the same time until it restarts.) This seems to have fixed it.
The changes to the keyboard are GREAT!
So for at least iPhone 5 and newer, I recommend upgrading. Because what I say matters so much.