Test-installed the leaked ISOs while waiting for the official release, to check for issues (deleted afterwards).
Using VMware 8.04 (744019) on Windows 7 x64, I could install both Windows 8 and Windows Server 2012 without trouble. Screenshots of both below.
Steps - select Win 7 x64 (I chose "server R2" for this test), and the install ISO, but don't "easy-install". Manually configure the VM, then boot and let the install DVD guide you through.
First impressions: The installer was very fast compared to Windows 7. I also installed Classic Shell and the Win 8 beta version of 7 Taskbar Tweaker (for those who like the classic desktop) and they didn't seem to have any trouble. First impressions were very favourable, although I hardly gave it a thorough test. My question was, as a heavy multitasker used to a finely tuned Windows 7 platform and Classic Shell, could I get a desktop interface that was productive for my workload on Windows 8 and Server 2012, and did I like how it worked when I did.
So far with a whole load of qualifiers (didn't test alternative file managers, apps, customisation, media or "real use") the answer is surprisingly "yes". They seemed fine, and Classic Shell did its job, though I can't test activation or updates as I don't yet have a proper license to activate with.
I can live without glass effects and "flip", and reports say the file manipulation and networking issues that caused dialogs like "calculating..." are fixed. That makes it worthwhile to me by itself. I haven't tested how Windows 7 interacts with Server 2012 (optionally with an ReFS-formatted server share). One new area of customisation will be needed if it matters to you - the desktop interface has lost its ability to set colours and element sizes (metrics) There's a "TinyWindowsBorders"/Regedit tweak for that annoying metric, others can be modified by registry or new tweaks/ customisations will be needed.