I haven't used the Keystation, but in general M-Audio's driver support for Mac OS is excellent - both my Midisport 1x1 and the Radium 49 I used to have worked fine even on OS9. However, unless you're going to use VSTs, you'd be stuck with Apple's built-in MIDI soundfont, which isn't very good (if not quite as terrible as Windows's.) Is there a particular kind of sound you're looking for? Do you necessarily need standard GM/XG, or just something that makes sounds? The MU-50 might be pricey, but you can get various MIDI modules pretty inexpensively (anywhere from $50-200) if you keep an eye out.