In iOS 8 and OS X Yosemite the iCloud architecture is changing. The previous "Documents in the Cloud" system is being replaced with iCloud Drive and the two systems are exclusive and incompatible although in most apparent ways they will function the same. Making the jump to iCloud Drive is a one-time transition and any devices running older operating systems (including iOS 7 and Mavericks) will become incompatible. You will be prompted to make this upgrade during your iOS update and need to consider whether or not it is appropriate.
- If you have any iOS devices that are unable to upgrade to iOS 8 then you probably do not want to activate iCloud Drive. At least until those devices are able to be replaced with current models. You will have to weigh up the problems caused (if any) should these devices lose the ability to sync documents.
- OS X Yosemite is not scheduled for release yet, and is about a month away. If you need your documents to sync with your Mac, do not activate iCloud Drive. When OS X Yosemite is available in a few weeks time you will be able to upgrade iCloud as part of the OS X Yosemite update, and this will be immediately reflected across your iOS devices.
You can perform the iCloud Drive transition at any time, including during the OS X Yosemite upgrade process. There is no way to revert this change so far as I am aware so you need to be sure you're ready. Having used iCloud drive for a few weeks I have only seen issues with a single app, but it's something to keep in mind if you have applications that you rely on. Checking with the developers regarding their iCloud Drive support may be important before you upgrade, even after both iOS 8 and OS X Yosemite are available.
If you do perform the iCloud Drive upgrade and find yourself in a position where your Macs cannot sync you do have a workaround. You can sign up to the OS X Yosemite beta, which has iCloud Drive support. This comes with its own risks, but that might be preferable to losing access to your documents.