I think in-game mouse sensitivity is affected by desktop mouse sensitivity. I'm not sure of this, but I know someone in my house keeps changing the mouse sensitivity and every time that happens I have to change my in-game sensitivities. Right now, my in-game sensitivity is 4. I know it's high by most people's standards, but I prefer to be able to turn around quickly. I've messed around with different numbers, trying to gradually decrease it like you guys have, but I always turn out worse and worse.
I use a sensitivity of 9.0. Most people use a low sensitivity (1.0-3.0) but I can't stand having to move my mouse half-across the desk to move the cursor. If you can "control" the cursor at a high sensitivity then it works great. And to answer Master_America, your Windows sensitivy DOES affect your in-game sensitivity.