First of all, Microsoft asks you to not perform an upgrade from the Windows 7 beta to the new release candidate. It's technically possible, but it's a scenario the company has not tested, and as such, it may result in bugs and other unexpected behaviour. They advise you to do a clean installation of the release candidate, but if you want to help Microsoft make the upgrade experience smoother, you can also opt to install Vista first, and then choose the upgrade option. The gathered telemetry could help in improving the upgrade process in the final release.
I did fresh installations on all my three Windows machines, and didn't find any problems whatsoever. All hardware in these three very different machines (a brand-new quad-core powerhouse, an aging Pentium 4 media centre, and an Aspire One) worked out of the box - I didn't even need to visit Windows Update. Still, it's advisable to do so anyway, because a few drivers did get updated that way. Windows 7 will mean the end of driver hunts after installing a fresh copy of Windows, and it has certainly closed the gap with Linux on this one.
One of the first things you'll notice in the RC is lots of new artwork and sound themes. They range from beautiful landscape photography to things I guess you'd see during an LSD trip. They also updated some icons, and included some new Aero themes for you to choose from.
Furthermore, the RC also comes with lots of tweaks and small feature additions, some of which were made at users' requests. The most important one is that changing UAC settings will now always require administrative privileges, a change instituted after several complaints on the internet about how you could turn off UAC and never see a confirmation dialog. Still, it doesn't solve the actual problem, namely that the default UAC setting in Windows 7 is insecure. I advise you to set the UAC slider to its top most position, as this is the most secure option.
The RC will be available at least through July, so there's no need to rush your download. Please note that your beta keys will still work.