After receiving an offer from Apple to upgrade to Yosemite, I did and my MacBook Pro was totally useless, I could do nothing at all, even couldn't start it. I had to get into disk utility immediately after switching it on, chose Time Machine and start again Maverick from there. Already upgrading from Mountain Lion to Maverick was a similar problem and Apple UK made me buy the Mountain Lion CD (that already was inside my MacBook Pro) to upgrade from there because the said that was no possible to upgrade from a Mountain Lion upgraded from Snow Leopard. Never ever again I will upgrade anything from Apple online. Full stop.