Just did a quick test, it seems stable indeed with 10.9.4.
I just did a single test and the results may therefore not be fully representative, but conversion of 66 D800 raw files with previews already generated was completed in 2:40 minutes with OpenCL activated (Auto mode).
The same task without OpenCL (never mode) took 6:20 minutes, so more than twice longer.
This is done on an 8 core machine with 64GB RAM.
This is great! Hope you guys can quickly enable the full usage of the 2 GPUs, that would be even better!