Like many people I have many different machines in the house and my Windows 7 machine upgraded without a hitch. When I say without a hitch, there have been a couple of blue screens and some odd behaviour, but it was installed a day after the launch.
An install on my Windows 8 Lenovo machine showed up the ugliness of Redmond launch in all its evil glory. I am not mincing my words because I have tried to do this install three times, and the last time was following Lenovo's instructions, so I am, to use an industry term, pissed off.
If you upgrade you are given confusing information and it is a lottery if your upgrade will hang.
First there is the confusing information. It is not clear when you take your media out of the drive and when do you put it back. The instructions from Microsoft are confusing and the whole idea of a multi-million dollar programming company insisting that an operating system need a user to sit by their machine to shove an SSD drive into the machine when it gets past the BIOS loading phase is just a joke. If you can't write a line of code to avoid that problem, then you have to question your ability to write something grown up like an operating system.