Installing from the Millennium edition disc (MM6 version 1.2) worked beautifully out of the box. Movies, sound effects all played. Game seemed to work fine, I specifically tested transitions between areas, shops, and combat/menu screens.
Running winecfg and turning off the two window manager settings in the Graphics tab was necessary to remove a close bar at the top of the (default) fullscreen mode.
This is on a Mac mini, Crossover Games v7.2.0.
Only thing that wasn't present is CD audio -- MM6 is an older game that uses a play disc with files in track one and then CD audio for the rest of the disc. Mac OSX doesn't seem to handle that the same way that windows does -- there are two different mount points, but wine doesn't have any provision to mix the two back together or directly mount/interface with the CD directly. Not a huge loss, but the music in this game is actually pretty good. That said, you could always just open up the second mount point in iTunes or something and play it along with the game.
Installing the unofficial 1.3 patch (available from http://telp.org/mm6/tavern/anyboard/posts/27430.html) works fine too (and I recommend doing so to fix some lingering game issues).