Here's big news: AMD has contributed patches to Cycles to make it work properly on their videocards!
This patch contains work to split the OpenCL mega-kernel into separate kernels to obtain better GPU utilization and therefore performance.
A description of the optimizations included in this patch is located here.
After many months of work, @AMD submitted a patch for Cycles to make it work with #OpenCL kernels. Milestone! https://t.co/2ckBfxCCzh #b3d
— Ton Roosendaal (@tonroosendaal) March 24, 2015