Don't hit that button yet! I know that everyone is eager to update to iOS 8 as soon as humanly possible, but when it comes to enabling iCloud Drive, you need to wait.
All the hype with the new OS has caused many to overlook the fact that not everything talked about during the launch will be usable today—iCloud Drive is one of them.
Check out the teeny tiny print right on the Apple iCloud Drive homepage, as it specifically points out that "some features require iOS 8 and OS X Yosemite."
Basically, any device not running iOS 8 or OS X Yosemite will not be able to sync properly to an iCloud Drive account. So unless every Mac you own is running the beta version of Yosemite, and every iOS device is updated to iOS 8, you do not want to update to iCloud Drive.
When you see the option above during iOS 8 setup, select Not Now and all your iCloud data will remain on your normal iCloud account, accessible by all your iCloud-enabled devices.
If you have already installed iCloud Drive, all is not lost. You can still upload photos and such to your iCloud account, they just won't sync with your other devices.
You will not be able to use iCloud syncing with devices not running OS X Yosemite or iOS 8, but you can still use your apps on the separate systems. For example, you can continue to use the Evernote app on your Mac running Mavericks, but your iOS 8 phone won't be able to sync that app until you upgrade to Yosemite.
So, if you need to sync your Mac with your iOS 8 device, install the Yosemite public beta (or dual-boot that sucker). Otherwise you will have to wait until later this fall when OS X Yosemite is released.