I recently updated to 0.8.4 RC1 NBI and got stuck at the apple logo screen with a spinning wheel and a STOP symbol superimposed on it.
I had to reinstall my OS using my old bootable usb which has a OS X 10.6 only on it.
I reinstalled it without erasing my hard drive and when every thing was done my system was back to prior to installing 0.8.4 RC1 NBI i.e 10.6.2 NBI 0.8.3 with all my apps, emails, documents, songs, movies etc intact.
Dont ask me how but thats what happened.
Maybe the senior/experienced members can shed some light on this?