Patrick,
I think many of the issues that Mac users have had to deal with are going to disappear with the release of the OSX version, because a lot of them are the fault of the Mac’s poor implimentation of certain things in OS9, like OpenGL and memory management. So far the realtime redraw in the OSX alpha seems real smooth. Plus you won’t have to restart your computer each time A:M crashes (like in OS9)! I agree with pequod with the slowness of classic mode.
A big drawback (to some) is the lack of Mac plugins. There aren’t many that are must haves, but I know I use Marcel Bricman’s Zevel plugin ALL the time (maybe a little less since the creation of the stitch tool). Thanksfully, Ken Baer ported Zevel over for us.
If you are looking at the G5, like I am, an OSX A:M and a G5 sounds like a great match. If you plan on staying in OS9 with A:M (which would mean no G5 for you!), I say just stick with a PC.
Jim