Valhalla Hills is running for me, with the Open Source AMD drivers. Yet, I have a Southern Islands card (R9 270X), which uses a different OpenGL driver (RadeonSI) than the Northern Islands based 6870 (Radeon), and I'm running the very latest Mesa and llvm versions (Mesa 11.0.6 and llvm 3.7 to be precise - llvm is only required by RadeonSI, it's optional for Radeon), giving full OpenGL 4.1 support with the open source drivers for Southern Islands.
If you are on Ubuntu (or Mint) you can check out the ~oibaf PPA, which packages the latest Mesa version (you'll need to google for it yourself, as the forums won't let me post links).
If you are, like I, using Gentoo, just do a world update. Mesa 11.0.6 has been marked stable yesterday (but you might still need to unmask llvm 3.7 for tesselation support).
From my experience the open source drivers perform slightly worse than the proprietary ones, but they are by far more stable.
The proprietary AMD drivers have at the moment four big advantages. The first is that they offer full OpenGL 4.5 and OpenCL 2.0 support. There are some programs that simply don't work with the open source drivers. The second advantage are application profiles (driver settings optimized for certain applications, and set based on the program name). Those are the reason certain games perform much better with the proprietary drivers. The third advantage is hardware support. The open source drivers work well up to Southern Islands. Newer chips are working meanwhile, but performance is still a bit low compared to the proprietary drivers. And last, but not least, they have slightly better power saving support, what might be relevant for laptop users (but see below, why I think laptop users should still consider the open source drivers).
The open source drivers have three other big advantages (four if you are using a rolling release distribution). They are much more stable. I had dozens of issues with fglrx, especially when it comes to multi-monitor setups. With fglrx one has to do every single change of the screen configuration in the AMD Catalyst Control Center. If one ever dares just to touch the monitor configuration tools of the desktop environment one is using, weird things start to happen (graphics glitches in some, but not all games, artifacts during video playback,...). And that brings us to the second advantage of the open source drivers, the desktop integration. As they are maintained as part of the Linux software stack (Kernel, Mesa, X.org), they integrate perfectly well with the Linux desktop environments. And this is also the reason for the advantage only relevant on rolling release distributions: The drivers are maintained directly upstream, so that they will work nicely with the latest Kernel and X.org versions, something that often is not true for the proprietary drivers (which, for instance, did not work with Linux kernel versions >=4.0 before last week). The last advantage is, that they support PRIME - on Laptops with switchable graphics one can switch between GPUs on a per application basis, while with the proprietary drivers one has to restart the X11 server to switch GPUs.
Anyhow, most of this will be irrelevant soon™, as AMD announced already some time ago, that in the future™ their proprietary drivers will use the same Kernel and X.org components that the open source drivers use - combining the best of both worlds.
