Kernel 3.11 just came out a few days ago. This is the kernel that's going to be used in Ubuntu 13.10 due out next month. As a Radeon GPU user, the best news is, of course, the dynamic power management for Radeon GPUs up to HD7000 series cards.
Even if it's not as good as Catalyst, Radeon display cards should perform much better than before. The downside is this new feature is considered experimental so it may not be enabled by default on Ubuntu 13.10. In that case, the kernel boot options will need manual tweaking to enable the new feature.
Basically, the open source driver will now be able to scale the GPU clock speeds and power so that you get performance when you need it, and energy savings when you don't. The current driver plays it safe and sets a fixed clock for the GPU which doesn't allow the GPU to reach anywhere near it's potential.
You can read the list of new stuff in kernel 3.11 from KernelNewbies.