I'm only just getting into OpenCL (I already know OpenGL very well) and I'd say OpenCL's programming language is closer to C than to GLSL. In any case, I'd say

I'm only just getting into OpenCL (I already know OpenGL very well) and I'd say OpenCL's programming language is closer to C than to GLSL. In any case, I'd say the hard part is parallelising your algorithms, understanding the memory model (private vs local vs constant vs global pointers) and using it and the the task dispatch mechanism effectively. Being experienced with OpenGL (or Direct3D, or game console) shaders will probably help for performance tweaking as the mindset is similar (memory access is evil, irregular memory access more so), but other than that I doubt it will help.