So far, I've been impressed by the performance.
Prey in OSX runs about as good as my 2.5 Dual G5 with a 6800U.
(How much better or worse, I'm not sure. I just played through a few levels again.)
Last night I installed XP via boot camp 1.3.
Supreme Commander ran beautifully. With the settings on high, I played with 5 AI on a medium size map. I expect it'll run just as well on the full size maps (especially with the new patch), but I haven't tried it yet.
Counter-Strike: Source performance blew me away. Even at max settings with 16AA and 16AF it ran at ~30 FPS. I ended up turning down the AA and the AF to 2x to keep the FPS above 60, but can't tell the difference visually. The HDR maps look great.
Sidenote:
I've been playing Supreme Commander on my roommates 2.66 Mac Pro (1GB RAM, 7300). With medium settings, this thing won't go above 15 FPS but won't drop bellow 10 FPS. The 7300 really does suck, but even at 10 FPS the game was 100% playable. Though the FPS is locked down because of a GFX card bottleneck, the CPUs, keep the simulation speed at 100%.