Assuming you are buying from reputable brands, such as Microsoft or Logitech, the primary culprit is likely not the mice, but rather the environment you are using them in. Optical mice rely on light bouncing off a surface and landing back on a sensor. If either the sensor or the light have been obstructed by dirt or debris,the mouse will not function properly. What can I say, 3 months or less of use, and you are telling me they can INDEED gather dust and debris? I must be a total idiot to have them go bad on me I guess.