At the very least, the mouse fatigues the forearm. Marvin Dainoff, director of the Center for Ergonomic Research at Ohio's Miami University, says the position for working most mice, with the palm of the hand parallel to the desktop surface, requires the user to rotate the forearm 90 degrees away from its natural bent-elbow position, which is the way you hold a cup. The designers are promoting mice they say are more ergonomically correct. Some of the design changes do make using a mouse more comfortable. Others improve the usefulness of a mouse, but it is not clear that they will do much to improve users' comfort.
Some companies simply jazz up the mouse with more buttons and icons.
The additional buttons allow users to scroll up or down a page of text without having to move the cursor to the screen's scroll bar. Take the wheel. Some makers turned the mouse upside down so that the user directly controls the trackball that moves the cursor. It's designed with a space so that the thumb can be used to brake the spinning motion. With the push of a button, the TrackMan can be set to scroll-wheel mode for browsing pages of text.