I've installed the RTM on several systems now and I have not encountered any problems. However, I'm doing clean installs on everything. I am not a fan of upgrading any operating system from MS if I can help it. There always seems to be some leftover cruft from the previous OS. At least with a clean install you know you're starting fresh.
This was especially true of my machines that were running Windows 7 beta code. There was some talk of the beta being upgradeable to the RTM. However, I'd be very apprehensive of doing that. If you're ambitious enough to have installed a beta OS, then you should be technically competent enough to do a clean install of the RTM. Take the time to do it right.
I have had very good luck with the new Windows 7 install, myself. Only hardware I have not been able to get a set of working drivers for is a TV/Capture card. But its not a major loss for me at this point in time.
I did a clean install to Windows 7 Ultimate 64 bit version. I was running Vista Ultimate 32 bit. And did a core install of applications I use most often. MS Office 2007, Visual Studio and addons, Firefox, and some other custom pieces of software. Everything seems to be running very smoothly on it.