Input devices like mice are very personal things: the particular fit of your hand, how you like to use the device, and many other things determine whether you prefer a mouse, trackball, tablet, or other input widget, and whether you want one button, several, or many. I have a whole drawer full of different ones, ranging from Apple's old ADB and pucklet mice to trackballs and other devices from Logitech, Kensington, Wacom, etc., all of which I've preferred at one time or another
The Mighty Mouse combines what I find to be the excellent size and shape of the one-button Apple Pro Mouse that preceded it with multi-button functionality and what turns out to be a very cool little mini-trackball in place of a scroll wheel. Not only does the mini-ball have a fun "grainy" feel to it as you turn (something I like in scroll wheels as well), it moves in two dimensions, and is also slightly spring-loaded to have a bit of "give." The resulting feel is something I find fits my work style quite well.
The multi-button trickery is a bit flakier, because there is only one main button on top: performing a right-click requires lifting your other finger off the left side of the mouse, so that the circuitry can detect that you intend to click the right side. The "squeeze click" and "centre-click" actions also feel odd. I'm getting used to each one, though.
Overall, I'd give the Mighty Mouse 7.5 out of 10. Not sure I'd buy one on its own, but if it comes with whatever my next Mac might be, I'd switch to it at home as well.