I went over to my local Apple store yesterday to check out the new mouse, and apparently it takes awhile for new stuff to get down here on the Gulf Coast because they didn't have any in stock. Bummer. However they did have demo units attached, er, associated with the display iMacs so I got to play with one. On the one hand, I would be looking forward to eliminating the occasional gummed up mouse ball with my current mighty mouse, on the other hand it eliminates two things that I really like: middle click and the fourth "squeeze" button that I use to open links in a new tab and Exposé All Windows respectively. You can use the 'secondary button', i.e. the 'right click' to invoke Exposé, but I already use the right click for the context menus.
Eventually (when they get them in stock next week) I'll probably pick one up, but the Apple person told me they really won't be supported fully until 10.6.2 which is coming soon® to use the new features. I didn't get one on this trip. I will say that it does match my keyboard and MBP, and the scrolling with the gestures is pretty cool, but I'm going to miss my "squeeze" option for Exposé.