Previous titles I always used 6, on this game ive been using 9. To anyone wanting to go higher but feels overwhelmed, just go up in intervals of 1. Also, you rarely need to use the thumbsticks all the way, obviously with a higher sensitivity you have the option to turn around super fast and check both ways when you enter a room, but when you are aiming down sight, you just slightly move the thumbsticks. Basically what im saying is if you can get used to a high sensitivity, you have a slight advantage, once you get used to the speed. You can precisely aim by being gentle and precise with the thumbsticks, and you can do a 180 and check side to side when you enter an area a lot quicker than you can with a low sensitivity. Just my opinion, of course, Hope this helps.
There are good points up there. Raise the sensitivity up by 1 or 2 points at a time. Learn to only move the stick partially.
Some I'd like to add including firing from the hip, which works quite well actually, and using the left stick for slight movement adjustments to aim. That one's very useful if you're only just a little bit off.