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Apple has recognized that 8.0.1 has some issues, and they have just put up a page that guides you through the steps to reinstall 8.0:
1. Make sure that you're using the latest version of iTunes.
2. Connect your iPhone to iTunes.
3. Back up your iPhone in iTunes on your Mac or PC. iCloud backups won't restore to earlier versions, including iOS 8.0.
4. Download the file below that corresponds to your device:
iPhone 6 Plus
5. Select the file you just downloaded by doing one of these in iTunes: Mac: Press the Option key and click Check for Update. Windows: Press the Shift key and click Check for Update.
6. Press Update to install iOS 8 on your iPhone.
The Health app won't work in iOS 8 after these steps. It will be fixed in our upcoming iOS 8.0.2 software update. http://support.apple.com/kb/HT6487