Pros: Great packaging, quiet fans, compact modular design, stable power.

Cons: Has had a distinct and noticeable intermittent coil whine since day one. I even went through the trouble of RMA with newegg to see if I got a bad unit but got the same results with the replacement.

Pros: same shape as the old grey intellimouse explorer 3.0 optical. good tracking on complicated surfaces. great placement of side buttons. no tilt scroll. easy to click scroll button.

Cons: you have to push in the "back" side button too far. this button is so bad that its almost unusable. others also note the clicky scroll as compared to the old intellimouse and this is true but it should smooth out with wear.

Overall Review: the "back" side button is so terrible that i feel its a deal breaker. if you spend a lot of time on the computer and use that side button a lot then look elsewhere. i ended up modding the mouse to fix the side buttons. i opened up the mouse up by pulling off the glued on glide feet and removed the screws. then i glued a thin piece of plastic (credit card thickness) to the parts of mouse that depress the side back and forward buttons to reduce the spacing. the buttons are fantastic now.

Pros: * Smooth ergonomics and large size which is good for larger hands * center scroll is smooth yet a bit clicky * 2 large side buttons

Cons: * Center scroll has a 3 way click (left, center, right) and the center click is almost impossible to press. You must bind left tilt and/or right tilts to a center click function to have it be usable. * Tracks poorly only complicated surfaces such as a "mouse rug" (google it) * Tracking sometimes went bonkers on the screen for a few seconds. * DIED ONLY AFTER 1.5 YEARS OF DAILY USE

Overall Review: I used to own one of those grey 5 year old Microsoft Intellimouse Explorer 3.0s. It died and I bought the MX400 because it resembled it closely but only had a lukewarm experience.