I've never used a Mighty Mouse and don't plan to buy one. $50 is far too much for a 400dpi mouse whose only special feature is 360-degree scrolling (though I do like that bit). I'm sticking with my Logitech MX518 that switches between 400, 800, and 1600 dpi on the fly, has forward and back buttons along with a middle click button I programmed to open stuff in new tabs in Safari, and has an extra button that I programmed to do the F9 function of Expose.
There is no reason to get the Mighty Mouse unless you place a large amount of value on the 360-degree scrolling and/or just like the design that much. Most multibutton (>2 button; 2-button mice with scroll wheel just work when you plug them in) mice have drivers for both Mac and Windows, and those that don't can be made to work as you deem fit through the freeware USB Overdrive.