Caitlin McGarry, writing for Macworld:
If you’re upgrading to iOS 8 on Wednesday, you must resist the urge to upgrade to iCloud Drive if you want to continue to sync your phone to your Mac. Why? Well, iCloud Drive only works with iOS 8 and OS X Yosemite. And you all know which OS we’re still waiting on[…] You can go back and upgrade at any time, but unless you want a file-syncing nightmare on your hands, you’ll wait for Yosemite’s official release.
If you make use of iCloud to sync applications or data between an iOS device and a mac, I would suggest skipping the upgrade to iCloud drive until you install Yosemite on your mac next month.