I got mine from MGS too. Squashed flat on arrival - which I ascribed to the 3000-odd mile journey across teh Atlantic.
As I have a shiny new laptop to check out as well I installed the game on 2 Macs within a few hours of each other. No issues at all.
I haven't tried multiplayer yet though, as there weren't any servers the one time I checked.
That wasn't much help, was it?
The only thing I have to add is that the game requires the DVD to be in the drive as well as the correct serial being entered, and it seems to be wise to the old 'don't want to scratch my disk so i'll mount a .dmg' caper.
I haven't tried really hard yet, but it looks like it would be non-trivial to duplicate the disk - meaning that a copied serial might not be too much use in itself.