I have a flashed 4890, so I was still using the 10.6.3 drivers in 10.6.4. When I installed the update (and fixed everything for the card), I'm seeing almost double the performance in all Valve games, and can now run AA with almost no hit. Portal with 8x AA now runs at a perfect 60, while no AA used to still stutter. Half Life 1 runs over 60 with 8x now, used to struggle to keep 60 even with no AA before.
Ep 2 still can't keep 60 fps while looking over the water in the beginning, but even Windows had trouble keeping that one up. I'd say it still runs better than it does in Windows without multicore rendering.
Pretty much no change in Starcraft 2. For some reason I can't get Torchlight to run in OS X right now. That would be an interesting one, pretty much solid 30 fps in OS X and 60 in Windows before. Really hope the update fixes that.