Interop between compute and graphical API is very important thing. We can render generated geometry directly on GPU without additional data copy through system memory. For example in cloth simulation we can do physics and calculate tangent space entirely on GPU.
There are some tests of interop and compute API performance:
GTX260 OpenGL + CUDA (Windows7 128 FPS)
GTX260 OpenGL + CUDA (Linux 143 FPS)
GTX260 OpenGL + OpenCL (Windows7 151 FPS)
GTX260 OpenGL + OpenCL (Windows7 167 FPS)
GTX260 Direct3D11 (Windows7 100 FPS)
HD5850 Direct3D11 (Windows7 214 FPS)
Same GPU shows 50% difference in performance across different API. That is nightmare.