I upgraded to OS X 10.10 Yosemite last weekend (from Mavericks). It was one of the top three most painful upgrade/install experiences I've ever had. (I don't recall the other two specifically, but I'll grant that this may not be the worst.)
I started downloading the installer overnight, and the next morning it had only downloaded about 100MB of 5GB. I had to go, so I paused it. When I started again a little later, it took several hours to finish.
The install was seemingly stuck for several hours with only 5 and then 3 minutes remaining. The weird log window indicated it was doing something with the many rubies I had installed via rbenv. I let it continue and eventually it completed. In total it took about seven hours to install, then it rebooted twice, then it screwed around for another half hour or so.
I ended up with over 114k log entries. Overall this was one of the worse OS install/upgrade experiences I've ever had, including Linux, but at least everything seems to be working.
When it was finally finished and I logged in, I was greeted with this scary dialog. It wouldn't go away, and I couldn't close it. I didn't recall ever using iCloud. I certainly haven't been paying for it. It never told me what files might be deleted. (As an aside, why would I even want to automatically delete local copies of files from iCloud? Obviously these are files I care about keeping around on all my devices.)
On the plus side, I can spell "Yosemite" without having to look it up now.