I just upgraded my 12" MacBook. Went smoothly with no issues. Yosemite 10.10.5 seems to make the graphics peppier. That is, the wait time for the password prompt when opening the lid, and any graphic 'stuttering' when using Expose seems to be completely gone. Dunno what they did, but it looks good to me. If this is a taste of the enhancements coming with El Capitan, we've got a lot to look forward to. Slowly but surely, even the small nitpicky things about the MacBook are being eradicated. Love this machine.