I experienced a glitch just trying to boot Yosemite... The loading bar loaded half way and just stopped. I let it go for maybe 15 min and it never loaded. I reset the SMC and PRAM, then decided to try and boot from the recovery partition (holding CMD+R) re-downloaded 10.10 and reinstalled. On it's final boot, it got to loading half way and once again it froze. This is on a retina MBP 13.3in with 3Ghz i7 and 8GB RAM.
I have a 2009 MBP 12.3 in with 2.7gh i7 and 8Gb RAM, and a smiliar thing was happening to my computer.
I installed Yosemite and everything was working well. Maybe a little slow, but that's understandable because it was updating all the apps, rebuilding my Mail database, iTunes database, etc.
Well I left the computer running overnight, and in the morning I logged in, but I was stuck on that loading bar, too. I waited for 30 minutes, and finally forced it to turn off and restart. It happened again, and I repeat the restart about 3x.
I finally reset the PRAM, and I got into the computer. Though it was still super slow.
Hours later, I go back to my computer to wake it up from Screen Saver, and I'm stuck at the load bar again. Reset PRAM again and now I'm back on...
But seriously. I'm having regrets about upgrading my older machine.