If you upgrade to iOS 8 tomorrow and you sync between apps on your iPhone/iPad and your Mac, you might want to hold off on upgrading to iCloud Drive. iCloud Drive only works on devices that run iOS 8 and Apple’s next desktop OS, Yosemite. Yosemite doesn’t come out until October unfortunately.
You can hold off on upgrading to iCloud Drive until your Mac apps have been updated for Yosemite.
I’m looking forward to using iCloud Drive once Yosemite finally drops. I’ve been moving away from Dropbox (using ownCloud for document syncing) and iCloud Drive will be a nice feature since I’m already in the Apple ecosystem.