With Blender 2.79, the OpenCL support has improved and should be closer to parity with Blender's CUDA capabilities. The OpenCL Cycles renderer has shorter render times by up to 50% in some cases, tiles are now seen updating while rendering, support for SSS and volume rendering, optimized transparent shadows, and various fixes.
Blender 2.79 Cycles is also bringing 10~20% faster performance for modern x86_64 CPUs with AVX2, light sampling optimizations, multi-threaded shader nodes compilation, brick texture smoothing, better multi GPU device handling, and other Cycles improvements.
Blender 2.79 also has automatic DPI support for determining HiDPI displays, an overhaul to Blender's video output support, and much more. More details on the other changes being worked on for Blender 2.79 can be found via this Blender.org Wiki page.