When you upgrade to iOS 8, you're asked if you want to activate iCloud Drive.If you're running other devices that aren't being upgraded to iOS 8, or if you have a Mac, it's best to say no for right now. Here's why.
iCloud Drive enables you to share files in the cloud. But iCloud Drive doesn't have backward compatibility with iOS 7, and on the Mac, iCloud Drive won't be supported until OS X Yosemite is available.
iCloud Drive provides a framework for you to share files in the cloud, and it's also how apps that depend on iCloud will sync their content. That's going to create some problems if you've got software that you depend on — and devices that you depend on — which can't all sync through iCloud Drive.
As an example, Nik Fletcher with Realmac Software posted a FAQ explaining the issue as it affects their Clear software, which relies on iCloud for syncing:
If you upgrade to iCloud Drive, you will only be able to sync with devices running iOS 8 or OS X Yosemite. As OS X Yosemite is still pre-release (and not yet available) upgrading to iCloud Drive will prevent you from syncing with Clear for Mac until both OS X Yosemite is released and you upgrade to OS X Yosemite.
Day One, the popular journaling software that runs on iOS and OS X, has posted a similar warning in their support forums:
As shown in the above screen, Apple's note is very important: "You will not be able to access the documents currently stored in iCloud on your other devices until they are also upgraded to iOS 8 or OS X Yosemite."
In other words, DO NOT upgrade to iCloud Drive if you want to continue to use iCloud to sync Day One on devices running iOS 7, iOS 8, and OS X Mavericks.
Similarly, Ginger Labs, makers of the popular note-taking tool Notability, warned its users that syncing will be disabled between iOS and OS X versions of its app until Yosemite is out:
Heads up! iOS 8 is available now. It includes a new syncing technology called 'iCloud Drive' which is not compatible with iOS 7 or OS X 10.9 Mavericks. If you use Notability on Mac and iOS, we suggest not upgrading to iCloud Drive until Mac OS X 10.10 Yosemite is released later this year. After OS X 10.10 Yosemite is released, you can upgrade to iCloud Drive through Yosemite or using the Settings app on iOS.
Bottom line: If you're only using one device, if all your devices have been upgraded to iOS 8, and if you're running the Yosemite Public Beta or a Developer Preview release of Yosemite and you're comfortable with how things are going, then you can probably pull the trigger on iCloud Drive safely.
But if you're still using Mavericks and iOS 7 on some devices and you rely on iCloud to sync data, hold off until you're ready to move everything up to the newest release. And for Mac users, that may be a wait of several weeks as work continues on OS X Yosemite.