Yosemite Upgrade on mid 2012 Macbook Pro

Performs well.

I haven't had any problems on several upgrades. You should do this in advance as a precaution:

Repair the Hard Drive and Permissions - Yosemite, Mavericks, Mountain Lion or Lion

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Repair

When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list. Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu. Select Restart from the Apple menu.

You can always try it on an external drive, rather than installing it over your current system. Whatever you decide to do, first make a complete backup of your existing system, so that you can revert if you need or want to.