Alan: have you had a chance to try this yet? How is it working for you?
I've played about a hundred turns on my MacPro, and I ran a few turns on my MBP 13". I can't afford much time to play a long game at the moment, and Civ5 is not a compelling "one more turn" experience for me. It seems to run OK, but I haven't played any big or long games.
Technically, the Mac version feels similar to my experience with the Windows version in Parallels 6, except I don't see the slow map redraws that happened in Parallels when panning, the leaders animate successfully, and there are no phantom terrain images of roads and oases. The main negative I have is that the terrain colours seem to be too saturated, so deserts look fluorescent, plains look like deserts, and snow is a white-out.