LOl.....I have played on 2  99% of the time... and can handle 3 for the most part ....I tried 10 (max) and well can you say blind drunk on a treadmill...... just pick a setting that is close to your liking and you will get used to it after a while......I have some friends who use the max setting .......and some like myself who dont.......

Go on Black Ops (if you have it) and play Combat Training. The sensitivity values of Black Ops are actually higher than in other games. e.g. 4 in BO is actually high, unlike other games where it is 2.

renessaince666 had the right idea. The sensitivity is really based on your own feel for it. Personally, I had to boost mine up alot higher just to feel like it wasn't dragging. Nothing more upsetting than knowing you're getting backshot because you can't turn around at the sound of gunfire fast enough.

I played on 2-3 all the time and was constantly wondering how people were turning so fast and strafing or flanking me and I just couldn't keep up with them. I watched some youtube vids and one of them suggested something and I tried it and it seemed to work pretty well.

Basically go into a private match with yourself. Set your sensitivity on 10 and just run around and goof off with it, then gradually bring it down until you can run around the map comfortably and you have a good feel for it. I wound up setting mine to 5 and I haven't looked back and my game play has improved quite a bit when lag isn't an issue. However when I get on the wrong end of the connection I still consistently lose every 50/50 gun fight.

From what I can tell the sensitivity change only seems to effect your turning speed.  The up and down motion seems to be the same regardless of where you set it.  I would suggest getting used to 2 and working your way up if you feel like you need to turn around faster.  For me I like 4 but I find sometimes I need to increase it when the game is laggy to get back to the same feel that I'm used to.  I think that I'm running a 6 right now.  I also have noticed that if I'm sniping (hardscoping) that I have to increase the sensitivity a couple of levels just to be able to track the enemy players movement because you turn slower when aiming down sight.

Also for getting used to your new sensitivity, go into a private match by yourself and practice spinning and hitting a target like a flower pot or something before going online.  Getting used to where to stop will help alot and it's a lot easier to learn that against a flower pot than a guy that's strafing back and forth while shooting you.

I play on 2 to 3 and I've used 5 before. The problem I have with the sensitivity system is the fact that I can't get things exactley where I want them. In the heat of things I jam the sticks and move faster than I need to. If I change it I usually get too slow. It would be nice if the developers would let you select sensitivity for each stick. This would allow me to move faster while keeping my pan/sweep slower.