One of the very important points is – if you upgrade to iCloud Drive in OS X Yosemite, you will not be able to use iCloud syncing for applications till iOS 8. And you can not undo the iCloud Drive upgrade. iCloud for notes, calendar, reminders and other stock applications work but iCloud for Tweetbot, Pages, Numbers etc. will not work. So, be sure before updating to iCloud Drive in Yosemite public beta program.