I have been running Windows 7 beta than RC for many months now. Despite all the warnings against using it as my main machine, I have done so, and have had little to any issues. The few things, such as the Bluetooth driver for my laptop, I have chalked it up to the standard driver delays for 64 bit drivers.
The RC has been reasonably rock solid. Yes, I still crashed once in a while, but much less than under Vista.