I felt the difference in my right wrist instantly. According to websites, "Ergonomic design keeps the arm positioned for natural, comfortable movement. It is clinically proven to reduce muscle strain and discomfort associated with Carpal Tunnel Syndrome and repetitive stress injury. The 3M Ergonomic Mouse is compatible using any USB port computer."
Ever notice that Gamers don't have carpel tunnel syndrome like most typists even though they likely spend more time at the computer? Well, 3M noticed! Your wrist sits on this ergonomic mouse like it does on a video game joystick. The weight of your hand is no longer carried by your wrist. Rather, your hand is relaxed on the mouse.
The relief in pressure on your wrist is instant. The cost may seem high, but it's less than a chiropractic appointment. Comes in 2 different sizes (small/medium and medium/large). This is a RIGHT handed mouse. You can program the left/right clicks if you are left handed, but the "joystick" is angled, so it's not as wonderful if you are a lefty.
Added note: The above review was written 2 years ago. It has been 3 years that I have been using this mouse. My wrist pain is completely gone. This mouse is so worth it. That being said, it is a P.O.S. as others have mentioned: the mouse does not travel well (it breaks if not protected in your computer bag) and breaks when dropped on hard surfaces -- I put a carpet under my desk and bought a glue gun to cut down on mouse costs. After gluing them back together, 3 have acquired the "double click" disease. Even with these drawback, I still feel that this mouse is worth it. I have purchased 8 over the past 3 years - I now keep one at work, one on my desk at home, and a "floater" for my home laptop that moves from room to room but does not leave the house unless carefully wrapped in it's original box. For "daily travel", because the 3M mouse has helped my wrist heal, I am now able to use a regular mouse, so I now keep a regular mouse in my backpack.