It is a complex pointing device. First of all, the most revolutionary from Apple, is that it has Two Buttons and a Scroll Wheel! The surely would silence all the Apple die-hards who continuously argue that one button is better than two, as Apple itself has given to the dark side of index + middle finger actions. However, its two buttons design is nothing like the alternatives from Microsoft or Logitech. It is actually achieved by having two separate sensors underneath the shell, instead of two mechanical buttons.
And the scroll wheel. Or maybe the scrolling ball. It gives you 360 degree of scrolling over large documents, which would be very useful for documents that need horizontal scrolls, like big images or large spreadsheets. That is almost two pointing device in one.
And the side button - squeeze to activate Expose. A bit like thumb buttons found on some others, now you can almost navigate around your Mac without touching the keyboard.
Despite its complexity, all these new features are packaged inside one simple, cute, egg-shaped shell, with no visible button at all from outside! That again proves Apple is not just a hardware+software enterprise, but a designing company!
But it is costly at AUD$79 from Apple's online store. Mac OS 10.4 Tiger required for customisation.
I think I'll keep my $19 Logitech OEM scroll mouse...