I am also experiencing this issue on our iPad Air since installing iOS 8. I am able to connect to a 2.4Ghz wireless network, although the down/up speeds are slower than they were on the same 2.4Ghz network running iOS 7.1.2. Connection to the 5Ghz wifi drops on the iPad after about 10-15 seconds (even while in the same room as the wireless router), completely unusable. However, an iPhone 5s on iOS 8.0.2 stays connected to the same 5Ghz network without an issue, and I am getting the same connection speed as before the iOS upgrade on the device.
Here is what has been tried to resolve the 5Ghz connection bug on the iPad Air:
Restarted wireless router and cable modem
Performed normal reset of iPad
Installed iOS 8.0.2 update
Performed hard reset of iPad (Home + Power button)
Reset network settings and attempted to reconnect to 5Ghz network
Tested with Wi-Fi Networking disabled and enabled under Settings/Privacy/Location Services/System Services
Reset iPad to factory settings
Attempted to roll back to iOS 7.1.2 through iTunes, but since Apple is no longer signing that iOS version it would not work