I bought that slightly before the iMac but using with it now.
It is a great "toy" for a good price, it works great with Google Earth and Google Sketch Up.
It does take a few hours to get used to and once you do it is second nature. It is the same idea as its predecessor, the SpaceBall, but it is a lot more ergonomic and therefore easier to operate.
On OSX it shows up as a joystick controller so you can use on games such as FlightGear.
I got the Space Navigator PE, which i cheaper than the Space Navigator SE. The only difference is the licensing, there is nothing to separate between them other than the sticker in the box. The SE has licence for commercial use and the PE desn't.