Receiving this error message. Even tried restarting my 6 Plus. Any suggestions?
My guess is that the servers are getting hit hard and this is going to be an issue until demand settles a bit.
There's going to be a demand spike as soon as it becomes available (19 minutes ago) which will last until the majority of those people actually download. Once that initial spike goes away, it should be much easier.
Right now, my iPhone 6 Plus is rebooting with the update, my iPad Air 2 is still downloading (I waited until my phone was done downloading before starting my iPad).