If you have an iPhone 4s and get a message to upgrade to IOS 8, DON'T DO IT. (been there and done it)
It will slow it down to a point where it's most annoying. And you'll be regreting having done so.
The upgrade requires over 4gb of space to install.
It is possible to downgrade back to IOS 7, but not for long. And this in itself is a Ball Ache, which includes using iTunes. EVERYTHING will be wiped from your phone, and it will be as you bought it. All apps, photos, files and music gone. Unless you have it all backed up.
If anybody wants to downgrade, go to www.gottabemobile.com/2014/09/17/downgrade-ios-8-to-ios-7 for step by step instructions. Apple don't tell you how. they have promised a fix, but nobody knows when.
I now have my IOS 7 back.