Yosemite install freezes on "Transfering your Information"
I had long wait as well.  Vaguely remember also at 20 minutes.  Also heard reports of similar waits on some of the podcasts that I follow.  I waited think a lot longer (than 20 minutes) and the install finished successfully.    As with all progress indicators, it is an indicator.   Sometimes things take longer.   In general would be very cautious to stop an OS upgrade in the middle.   almost certain to have things screwed up.