they did a shitty job of porting it. it's difficult to run, which is pathetic considering that it's a mere port and they took forever to get it on the market. FPS isn't good either. the graphics are nothing compared to what will be offered by the new doom and half life games, and farcry. i've played this game about 70% through so far, and i wouldn't give it higher than 8/10. the indoor levels are some of the most generic crap ever.
i recently picked up Farcry on the recommendation of someone on another board, and it beats the shit out of Halo. incredible outdoor environments, cutting edge graphics. it's a hog, of course, but i plan to be running it on 1600 x 1200 with everything turned on next week(geforce 6800 ultra).