After OS Yosemite was installed in some of our MacBooks, we had some boot at 50% and it stays stuck there. The first time I saw the issue, I power cycled the MacBook and I let it sit for an hour. It was still stuck at 50%. I power cycled the machine again and left it on for the whole night (5:00pm-8:00am). As I got back to the MacBook, it was still stuck at 50%.
I did some Google searching and I read this thread here: https://discussions.apple.com/thread/6603394 and tried booting to the recovery partition (Command \+ R). I booted to the recovery partition, turned off Wi-Fi, then restarted the machine using the menu bar.
After doing this method, the machine successfully rebooted and finally reached the login screen. I have done the same methods for another MacBook along with one MacBook Pro and they all managed to reach the login screen after booting into the recovery partition, turning off Wi-Fi, then restarting the machine.
I don't know if this is the "right" solution for this issue, but other ways to solve this issue would be greatly appreciated as we are planning to upgrade more and more MacBooks (and Pros) to OS Yosemite.