I suppose that I might as well add my own very frustrating experience. I upgraded one iPhone and two iPads while on my own network at home with no problems. All of these took 30 min. to 60 min. to download on the first day of availability. We have been trying to upgrade our other device, an iPhone 5s, since the launch date for iOS 8 with no success on the same network connection. We typically get estimated download times of 24-32 hours. Even with that, the download never finishes. I have tried with iTunes as well (on a separate computer), with the same non-results. At the moment I am trying to download the file by the direct link. It has been running for three days now. It continues to pause with a timeout message from time to time so that I have to resume it. At the moment it reports about 6.5 hours to go. Yesterday at this time it estimated about 8 hours to go.
For the record, I tested my connection and it reports download speeds of about 20Mbps. All other files that I have tried to access or download for the last few days have had no problems at all. So this appears to be a problem with either the servers or Apple's content distribution system. Does anyone know a way to ensure a connection to one of the more efficient download paths?