It's on the black apple logo screen with the white progress bar around 90%. It hasn't moved for a couple hours. Hard shutdown and restart just puts it right back there. Command-R boot does the same thing, no effect.
Well, sure….
But did you notice how much faster it installs than Yosemite? I remember a lot of posts here where early Yosemite wouldn't get past 80%, so 90% on El C. is much better, I think.
So, maybe the install came with special notes, you know, for the developers that should be trying it out.