I've purchased the Steam version of this game and have racked up almost 900 hours in designing and upgrading transit routes in Russia, France, Italy and the U.S. The modding community is very active and has made patches and mods in order to correct some of the few faults found in the game (low capacity vehicles for one).
Though the hardware requirements demand 512MBs of VRAM, some Macs can handle the game with only 256MB depending on your video card (ATI versus Nvidia). If you have low VRAM, turn off HDR, filtering, v-sync, anti-aliasing and set shadows to low. Just expect gameplay running at 8-20fps at fullscreen 1080p resolution. Because it's a sim game I've accepted the low framerate.
If you have an older Mac but really want to try this game, it IS possible but there are caveats. If you're willing to take the plunge the game is rewarding, has tons of support and lots of DLC. Highly recommended.
My Mac's hardware:
2.0 GHz C2D, 500GB 7200rpm Hitachi TravelStar HDD, Nvidia GeForce 9400M 256MB, 8 GBs DDR3 1067 RAM, Mac OS X 10.8.2.