With the pending release of iOS 8 on September 17 and the subsequent availability of iCloud Drive as part of the upgrade, fair warning from some of the development and Apple press community.

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.
Macworld

As iCloud drive requires iOS 8 (let’s assume you will be upgrading) soon to be available, and OS X Yosemite which is currently not available, Mac and iOS devices will not sync through iCloud Drive if you are running an iOS 8 and OS X Mavericks (or earlier) combination.

Therefore, when upgrading to iOS 8 – select the Not Now option when confronted with the iCloud Drive screen during the installation process (once Yosemite is on your Mac, iCloud Drive can then be activated on all your devices).

Don’t use iCloud or don’t own and sync with a Mac? – upgrade and embrace iCloud Drive until your heart’s content.

Read more from two developers here:

FAQ: iOS 8 & iCloud Drive
iCloud Changes in iOS 8