I'm currently (at the time of writing) in the process of upgrading Windows 7 to Windows 8 (17% complete, so far). Although the setup routine told me that I cannot use my computer while the upgrade is in progress, it's quite easy to start applications by hitting the Windows key or just using the second monitor.
I'm aware that the setup routine might (force) close my running applications and reboot at any time. I know that, and I know that using the PC under these circumstances is most probably "not officially supported". Are there any other real-life problems to expect when using the PC while the upgrade is in progess (apart from the potential loss of unsaved data due to the machine rebooting at an inconvenient time)?