Ultimately it's about what you are used to and comfortable with. I play with an above average sensitivity, but can never go too high or too low or it just feels wrong. I'm always one of the better FPS players in any game I play and my controls definitely do not hinder me.
You'd be surprised. A long time ago someone suggested I try a really LOW sensitivity in ET, and I never believed it because I was shooting fine. I tried it, and my accuracy in matches went up on average 5-6%, and was hitting MUCH more headshots. I still use high sensitivities in most games, especially those that dont have head hitboxes. Like if I were to try a low sens in Quake3, my rail would go up (although it already blows) but the rest of my weapons would suffer.