"The implementation of OpenCL is only in an experimental stage and disabled in official builds."
Quote:
"OpenCL support is still being worked on and has not been optimized as much, because we haven't had the full kernel working yet. "
OpenCL builds on Graphicall are a bit old (2.61 ish) and descriptions would seem to indicate that it is very unoptimized, being equal or even slower than CPU rendering at this time.