I have done this twice and still have the issue on an Intel based iMAC 27"
First time Yosemite froze at the grey apple logo screen with progress bar at 1/3rd. A Re-install sorted that and things worked great for about 48 hours
Now it will pass that screen, go to a full grey screen with no logo and (a) either freeze there (every now and then) or (b) wait there for near to 5 minutes before presenting the user login screen
At the user login screen the system waits (spinning wheel going full bore) for another 5-10 minutes before producing the login prompt (window where you can type your password after picking the login account)
Password entry completed it is back to spinning wheel and waiting while it apparently processes this. This can take another 10 minutes or more....and usually results in the screen going black with only a mouse pointer visible. The pointer can be moved around but clicking on it does NOTHING, and the system sits there indefinitely.
I have done the Safe mode stuff, the NVRAM/PRAM clearing stuff, the Disk Utility repair and permissions repair, Recovery and as I stated - have re-installed 2 x aleady.
Very strange behaviour, and as one would have it, this all started the week after my 3 years of warranty and apple-care expired (go figure)