The Mighty Mouse is mighty fussy. Apple sent me a review unit last week, and in our testing the mouse falls short in several regards. Most obviously, I continue to find the overall shape of the mouse ergonomically unsatisfying, but I have hand and wrist problems that make a regular mouse uncomfortable. (For a general description of the Mighty Mouse, see "Apple Ships a Multi-Button Mouse" in TidBITS-791.)
First, the scroll ball (what New York Times columnist David Pogue calls a trackpea, a term I like) is not a revolutionary breakthrough that puts shame to all other scroll wheels. It's a tiny, hard-to-use ball that makes a barely audible ticking sound (generated via an internal speaker) as it's used. I found it tricky and no improvement over a scroll wheel.
The left-right touch-sensitive clicking works fine, but it's not worth crowing about. But I have no complaints about two physically, mechanically separate buttons either, making Apple's design mostly of interest for the way it can switch between one button for those who prefer simplicity and two buttons for those who want more flexibility. However, TidBITS Managing Editor Jeff Carlson found the touch-sensitivity to be tricky, because he often rests his index and middle fingers on his two-button Kensington mouse; using the Mighty Mouse required that he either suspend his middle finger in the air above the right button (quickly creating a sore finger) or move it off to the side.
Squeezing the mouse to activate the two side buttons seems to be a particularly strange action, versus pressing a single button, and the addition of extra buttons doesn't solve any problems for me.
I also find the Might Mouse software (which installs from an included CD) confusing. Plug in a Mighty Mouse without installing any software on any platform (Windows or any Mac OS X release), and the main left and right buttons work by default. Install the software for Mac OS X 10.3.9 to 10.4.1, plug it in, and the left and right buttons work. However, install the software for Mac OS X 10.4.2 or later, plug in the mouse, and you get only a single big button at the top, requiring you to enable the multi-button functionality manually.
Another shortcoming, Jeff noted, is that you can't reprogram the right-button action. He uses a right-click as a double-click (which I find mystifying, but each to his own), but that's not possible using the Mighty Mouse software, unlike the commonly used Kensington MouseWorks (for Kensington pointing devices) or Alessandro Levi Montalcini's $20 USB Overdrive utility (for nearly any USB controller), neither of which dictates particular actions mapped to particular buttons. USB Overdrive 10.3.9 already appears to work with the Mighty Mouse if you don't install Apple's drivers, and Alessandro has committed to supporting the Mighty Mouse fully in future releases.
Overall, Mighty Mouse doesn't measure up in design and function to many other mature mice. Its features are unique, but not compelling.