The SpaceNavigator is a standard multi-axis controller HID device. It will be detected properly by OS X and you should be able to access it using the HID Manager for instance.
One nice thing with our driver is it will allow you to control the speed and axis transformations even though you're working with it through HID. Just add your application to the 3Dconnexion preference pane.
The driver will work with multiple devices plugged into the same machine. However, the preference pane does not differentiate between the various devices. Any setting you apply will be used for all devices.