I’m another mac user and I feel it’s appropriate to update this thread and let potential mac users know the OP’s concerns have mostly been addressed by Red Marble, the company handling the mac port. As of this posting, all expansions except Nomads are available for the mac, load times and graphics both seem to respond in an utterly reasonable manner, and on 9/12, patch 1.46 was released to mac users, including a fix to the game’s auto-updater to supply future patches via Red Marble instead of Positech, the upshot of which is that patches aren’t unusable .exes anymore.
The mac port seems to be about a month behind the original version of the game in terms of new content and patching, but I’m sanguine enough about waiting. My only real complaint about the port is that, on my MacBook Pro, with Snow Leopard 10.6.4, the track pad multitouch seems to cause huge instability; the game often doesn’t respond to clicks at all, and two-finger right clicks on the deployment screen cause a crash more often than not. There is a very easy workaround for this, however. I just plug in a mouse and everything runs fine.
As to modding, I’ve found it takes about ten minutes worth of work with GraphicsConverter to change all those .dds files to .pngs, and the data files that refer to the graphics don’t need any alteration at all; somehow the program seems to obtain .png files of the same name even when the text calls a .dds. If none of that makes any sense to you, this is something you don’t need to worry about at all, and if it does and you’re thinking about installing some mods, grab graphics converter and the unarchiver (to get self-extracting archives or .rars to open) and you’re good to go.
So, “friggin’ terrible?” Not really. Just a little more work for modders and a little slower to release content. If you’re okay with those things, I’d absolutely recommend picking it up, because this is a really great game.