Running Windows 8 Pro x64 Retail on a 15" rMBP nearly flawlessly. I installed from a flash drive created with the boot camp utility; I encountered no problems during setup, and the OSD for the brightness and sound adjustments does show properly. The only hangup is the fact that the boot camp control panel has permissions issues and won't load under an administrative account, so you have to make a secondary admin account, change the original account to standard, log in to that, adjust the boot camp control panel settings (for some reason it has no problem working with more restrictive privileges), and then give the account administrative privileges again (because boot camp control panel settings are specific to each account). Or you could install trackpad++ or something.
Of course, that's all notwithstanding the infuriatingly crippling lack of integrated GPU support under boot camp. I really hope somebody can figure out how to get a fully-functional native EFI installation working soon.
Had some problems with my new rmbp 13.
First boot was completely messed up, it couldn't display properly on the notebook screen. Plugged in an HDMI monitor and got the apple drivers installed. That fixed that problem.
After I did that, the trick was to try and get windows update to run before the thing crashed. There is an update that needs to be applied to stop macbooks from randomly freezing.
Finally, installed TrackPad++.
It all works well now. I do find that Apple and MS ecosystems do not play well together. That is, my media is quite trapped in one or the other, and that is lame. Means I'll be picking one, and OS X will win.
You could always install a tool to allow one OS to write to the filesystem of the other, and as you probably know, with a boot camp installation both OSes can read each other's file systems without any problems. Also, protip for running Windows at native retina resolutions--under screen resolution and "make text and other items larger or smaller", if you use a custom setting of 199%, everything looks pretty nice without some of the interface elements bugging out like crazy if you go to 200% or higher.