I have downloaded it and installed it on my iPhone 5 and iPad2. The installation on the iPhone was straightforward and relatively quick. The iPad2 installation took longer but completed without a problem however with iOS8 it is slower opening apps and appears sluggish. Perhaps a step too far for the iPad2.
Having finished the updates I had a very large number of app updates to do. Some like Pages and Numbers took time.
I have started a thread in Tech Helproom which may have been better here or deleted having found this thread.
For Jock1e - Messenger is not part of iOS8. Both Facebook and Messenger apps updated after the iOS8 install.
Facebook works. All that has happened is that Facebook have moved messaging to the separate Messenger app. If you click on Messages in the Facebook app then Messenger opens. It has an advantage in that if all you want to do is send or receive a message then you do not have to use the rest of Facebook. It is not a problem as such just a different way of doing it.