Just a warning - after 6+ hours (not sure exactly as I went to sleep) my Yosemite install failed because the log buffer was full. I can't say for sure that this would not have happened if I hadn't left the log viewer open, but I would recommend closing the log window when you're not actively using it.
This time I only opened the log viewer briefly at one point during the upgrade, and again after several hours because it seemed to have hung again. I can't imagine I'm the only one who's affected by this issue. I'll report back when/if I find out how to fix it.
In the meantime I've restarted the process one more time. The laptop still boots straight into the Yosemite installer so I don't think the hard reboots have adversely affected the machine.
edit: It occurs to me that the longest I've waited has been 12 hours (my first attempt). I've now seen reports of 15 hour installs, perhaps I didn't wait long enough? Even if the log buffer is full, that wouldn't necessarily mean that install has outright failed... hmm
