Dealing with ergonomics specifically, the Magic Mouse shines in all fields. The five areas that compose ergonomic research are safety, comfort, ease of use, performance, and aesthetics. There is no one field in which the Magic Mouse lacks any of these areas. This is why I consider the mouse well designed.
The Magic Mouse is definitely safe. It has a smooth rounded top and lacks any sharp edges. It would be quite difficult to cut yourself on this mouse. It runs on two AA batteries that lack enough juice to electrocute you so that is not a concern, not that electrocution by computer mouse is all that common.
Comfort is evident in the Magic Mouse. It has a glossy finish on the top that your hand can smoothly scroll over. It is wireless as well so you can use it pretty much anywhere. Be it on your couch, bed, chair, floor, etc the mouse functions on almost all surfaces. It's round shape fits well within your hand. There is no one thing that makes this mouse uncomfortable.
Ease of use is one of Apple's calling cards. The Magic Mouse is no different. It is identifiable by its relative size and shape. It is very straightforward in its use. Just point and click like pretty much every other mouse out there. The wireless Bluetooth adapter is built in and is very easy to connect to your computer. Even changing the batteries is simple. You push one button on the bottom of the mouse and a cover pops off exposing the batteries.
Performance is on par with four different wired mouses that I have tried. It is good for fine selections in Photoshop and surfing the web. It has features built into the mouse that allows the user to navigate web pages easier. It has a swipe feature that lets the user go back and forth between web pages. Scrolling down on the mouse also scrolls down web pages as well. Clicking is very intuitive and smooth because the mouse doesn't have individual buttons. It is a monolithic design. My only gripe about the design is the symmetry of the mouse. Sometimes I grab the mouse upside down and it works poorly. They have an apple logo to help orient the mouse, but something tangible would have been considerate.
Aesthetics are always apparent in Apple's designs. The Magic Mouse is white and grey. It has a glossy white finish on the top of the mouse and a half inch wide Apple logo where your palm holds the mouse.
Apple is known worldwide for good ergonomic design. Their products create a buzz because of the way it looks, how easily people can interact with it, and originality in design. The design process at Apple is comprehensive and deals with most constraints.