sorry to chime in so late on this topic, i've made 5 mame machines runnin macs with os 9 and earlier. of course both usb gamepads and the usb ipac work great for this. after much testing and troubleshooting it is just easier to go the ipac route. sometimes when you have more than one usb gamepad plugged in they will switch places in the usb chain when you restart. its very random and very frustrating, player 1 and 2 swap, its enough to drive you crazy, however if you can get by with only one gamepad your golden, more than one go with the ipac.
edited for terrible spelling sorry....