In short: When you upgrade your iOS devices to iOS 8 this week, we highly recommend you wait until Yosemite ships to upgrade to iCloud Drive!
With iOS 8 Apple will transition to iCloud Drive for syncing documents between your Mac and your iOS devices. iCloud Drive is a great new infrastructure that is a lot more robust than Documents in the Cloud and it will resolve a lot of iCloud syncing issues users have experienced in the past.
As iCloud Drive and Documents in the Cloud are two completely different technologies they do not interoperate! When you upgrade to iCloud Drive all your documents will be copied from the “Documents in the Cloud” servers to the new “iCloud Drive” servers. As iCloud Drive is not supported by iOS 7 (and earlier) and OS X Mavericks (or earlier) this will mean that when you make changes to your documents on iOS 8, you will no longer see those changes on your “older” devices.
In other words, when you upgrade to iCloud Drive, you will get two different sets of documents. Documents that sync between your iOS 7 devices and OS X Mavericks Macs and documents that sync between your iOS 8 devices. OS X Yosemite is able to handle both.
We recommend you wait with the iCloud Drive upgrade until you upgraded all you iOS devices to iOS 8, and until you upgraded all your Macs to OS X Yosemite (which won’t ship until later this year). So when you upgrade to iOS 8 and the setup assistant asks if you want to “Upgrade to iCloud Drive”, we recommend you tap on “Not Now”. If you don't own a Mac and plan to upgrade all your iOS devices to iOS 8, you can already upgrade to iCloud Drive.
As this transition is managed by Apple, we are unable to work around the issues that might occur during the transition period. Please make all your colleagues and friends aware of this important transition!