It's pretty... - the maps are the same as the PC version, of course, but the controls look a little cleaner. Not completely easy to use with a touchpad, but I just need to get the hang of it a bit more. A bit jerky at first, but that's the maps not being cached yet more than a speed problem. However, this is a 1.2 ghz G4 with 1.25gb RAM, with slower machines YMMV.
New icon in its honour - it's the PC version though since I'm not 100% sure how to do a Mac screen shot.
Later: Just to clarify things, there's been a Mac version for a few weeks but it only ran on the latest version of OS-X; I tried it, and it simply wouldn't run on anything earlier. This one runs on the last version, 10.3.9, so you don't have to upgrade to Tiger to use it. And that saves about a hundred pounds.
Not sure if there are any other changes, but anyone who got the earlier version might want to check.