Good news for the people that still have to run Windows apps but want a secure and stable system without having to understand how security works: QubesOS R2 is now deemed stable for daily use, even though still in beta.
QubesOS is is fedora-based operating system that now also supports all features in XFCE next to KDE, and provides a very easy to use full application-level virtualization and group-permission automation with the emphasis on a "seamless" GUI experience. This is implemented to a very high degree of comfort, in that even cross-platform fully secure drag and drop or copy and paste operations are supported, and that now, automatic starting of applications in a disposable virtual container is supported.
It requires a pretty modern system (at least dual core and less than 5-6 years old CPU), preferably with IOMMU (VT-d/AMD-Vi) support for maximum ease of use, performance and security, with enough RAM (at least 8 GB of RAM is recommended if you're going to use Windows applications).
The virtualization is based on Xen, and the system uses colour coded windows to indicate the security level to the user. The user can run windows applications in a dedicated windows application virtualization windows just the same as normal applications, without having to switch to a vitual machine or without having to make a clear distinction between native and windows-applications, which makes it very easy to use for people that don't want to break free of previously acquired windows-habits, but still want a system that they can depend on.
I haven't tried it out yet, but I do believe that if the developers say it's stable and fit for daily use, that it's actually true, they have a shown a pretty solid development parcours, and are not fooling around.
The Windows application virtualization is of course based on proprietary code, but this code is provided free in QubesOS for as long as the developers aren't entering the final commercial phase of their product.
I would definitely recommend QubesOS to users that are running Windows and are satisfied with the performance of Windows on their hardware, but want to switch to a secure and stable system to get rid of those nightmares and regain their lives. I would not recommend QubesOS to users that are experienced with linux distros that use SELinux or Tomoyo v1, and that can configure their system, or that are used to linux performance on their hardware, because QubesOS is obviously heavier than a standard fedora install, and is only about as fast as Windows 8, because of the extra overhead of individual application-based virtualization. For those advanced users, QubesOS will probably not bring much security benefit anyways. For those that have advanced to the level of mostly using the CLI to get things done and truly understand the security mechanisms of modern linux distros, QubesOS is probably not recommended at all, as the colourful windows and permission dialogues will probably get on their nerves, and the GUI management of permissions will probably feel like obfuscation, but then they already know all of that stuff and won't even consider trying it anyway lolz...