This is a mechanical (not optical) trackball, and they made a critical design error: the rollers are metal, not rubber. This means that if there's any stickiness to the rollers at all -- say, if dirt or hair or pet fur gets in them -- the ball slips over the rollers and it ceases to function. And the rollers aren't easily accessible so they're nearly impossible to clean.The trackball is nice and big, which makes for good control (great for gaming or detail graphic work) but after a month or so of use it simply stops functioning well. It just isn't worth the price.