I got the blackscreen of death after the first reboot while installing the upgrade. Seems that the nVidia driver (v3.54) they packaged with Win 10 was a beta version and for some reason - at least with Alienware X51s that have an nVidia card, the Win 10 installer assigns your monitor to the Intel onboard HDMI connection - even if its disabled in the bios '>
Apparently, the solution is to enable the onboard HDMI port, unplug your monitor from the video card, plug it into the onboard HDMI port, then install the v3.53 driver from the nVidia website.
If you have really bad luck like I did, the installation appears to have crashed before you discover all this, and like a noob you force restart the computer and actually do crash the installation. This forces you to wipe the computer and reinstall Windows 8.1, eventually having to go through upgrade hell all over again.
However, it was a good excuse for me to buy a bigger, HDMI enabled monitor... '> As soon as that comes in, the whole computer is going to the local computer store and they can do the upgrade.