I have no sensitivity setting because I am an insensitive pig, or at least that is what I have been told but then again I could careless what those people think....
I've got a Microsoft Sidewinder mouse... but only because I like bulky heavy mice. I keep it at 800DPI, but set the game sensitivity to 15. I started at 3, then 5, then 9, 12, finally 15 as I play the game.
But I can't put the DPI higher on the mouse, or increase the sensitivity much more than that before I loose control of the mouse
I usually play with 1800 dpi and 10.5 (TF2) settings, though if I'm trying to snipe I'll usually take my dpi down a little bit for slightly better precision... not that it helps that much, considering I suck at sniper. -_-