AMD endeavored to improve the Cycles ray tracing engine in Blender. The lack of performance (or even outright failure) has been a thorn in the side of the Blender community for a long time. We've been working to remove that thorn.
I wrote a blog about the effort here: http://developer.amd.com/community/blog/2015/07/10/collaboration-and-open-source-at-amd-blender-cycl...
The code was submitted for review in March, and the reviewed, modified,and approved by the community. The new OpenCL kernels for Blender Cycles are available in Blender 2.75. It works, and is quite a bit faster. Read the comments on the blog, there are a couple of real-world reports. There is good work behind this, both from within AMD and the Blender developer community. IMHO, goodness all around.
This is super-fantastic news! I hope we will now have a thread singing the praises of this new patch - equal in length to the original thread bemoaning the problems.
I tested 2.75 beta version. In my machine (Core i5-3470 ~4.0GHz, Radeon HD7850 1GB, core=1050Mhz, mem=1245MHz) Cycles on the GPU was nearly 2x faster in GPU than CPU (in the best case). My conclusion: it is usable but it can be better.
Thanks for the feedback. I'm not sure what that team is going to work on next, but additional work on Cycles is certainly possible. I'll make sure the team hears the feedback.