I don’t like Apple’s Mighty Mouse. Aestically, it looks very good. Size is pretty much a preference thing so I won’t comment.
However, the scroll button feels flimsy and due to the touch sensitive buttons, some of my clicks don’t register, which can get very annoying. This also means that the buttons don’t work independently. I currently use a Microsoft Intellimouse with the tilt wheel. I have the middle click button setup to work expose the Finder desktop via Exposé. When I want to drag a file from the desktop to an application I’m working in, I click the middle button to expose the desktop, left click and drag the file in question and then click on the middle button again to return to my application while still holding on to my left button. This allows me to drop the file into my application. You cannot do this with the Mighty Mouse, which is a shame.
I also prefer the scroll wheel on the Intellimouse than the scroll button on the MM. The Mighty Mouse’s scroll button feels flimsy and cheap. It’s also harder to control than the scroll wheel which seems to have a nice “drag” to it. However, side scrolling is better on the Mighty Mouse as the one on the Intellimouse does not scroll continuously horizontally.
Of course, the Intellimouse is also cheaper. ;p