If you do a lot of business travelling, or if you work primarily from a laptop system with external peripherals, a good mouse may make you more comfortable and productive. Portable (laptop) mice have changed much in the few years that they've been widely manufactured and marketed, but their usefulness has remained more or less the same. Microsoft made a notable effort to include as many interesting features in the Mobile Memory Mouse 8000 as possible, resulting in a mouse that can much more easily convince you to abandon your trackbutton or touchpad for extended laptop computing sessions.
Microsoft Mobile Memory Mouse 8000 review