I upgraded to Yosemite in the day when Apple release its first DP, on my primary laptop unfortunately. I have to say the hand-off feature is really impressive, but in the meanwhile I was suffering from frequent system crashes.
When I decided to downgrade my Macbook to Mavericks, I find out it's not that easy.
After some googling I found out a painless way to do this.
Step 1. Backup your system
At the very first, make sure you have already backup your documents by Time Machine.
Step 2. Get Mac App Store ready
Launch the Mac App Store, navigate to page of OS X Mavericks. DO NOT click the download button here, or you'll be prompt kinda "OS X 10.10 has already been installed.". Just leave the window open and we'll get back to here very soon.
Setp 3. Update the system version number
Then open this file
/System/Library/CoreServices/SystemVersion.plist using XCode (or other apps which could edit a plist file, like TextMate). Open it and find two string of current version number
10.10. Cause we're gonna to downgrade to 10.9 Mavericks, to ensure the download could start we change it to
Save the changes and enter your password when you see the dialog.
Step 4. Download OS X Mavericks & install
Now get back to Mac App Store, click the download button of Mavericks. After entering your Apple ID it would start downloading. Just wait it to finish then the installer will automatically start.
Now the only thing you need to do is to complete the install process as usually. All your settings, documents will remain in your system.