DesertFox,
I have to disagree with you, at least from an FPS standpoint. RTS or RPG games like StarCraft, and Diablo are just fine with optical mice. However, an FPS game like Half Life requires the player to be able to execute fast-twitch motions. Many times, the difference between life and death depends on you being able to do an immediate about-face to kill your enemy behind you.
Optical mice cannot do this. If you flick the mouse to the side too quickly, the laser cannot "read" the texture of the surface quickly enough to register your movement in the game. The result is that you only turn 90 or some odd degrees, rather than the full 180 degrees you need to do an about-face.
If you don't believe me, try this test. Place your mouse cursor all the way to the right side of the screen. Slide to the left VERY QUICKLY to get to the far left side of the screen. You will see the cursor stop moving halfway. Now try the same test with a ball mouse, and you will see that you can get to the other side with no problem.
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COUNTER-TERRORISTS WIN!