My Sensitivity Level - 4-7 With Assault Rifles, 9-10 With SMGs and Snipers.
Why - Keeping a lower sensitivity with the assault rifles just feels better, I feel like I can be alot more accurate and I dont need to be able to turn on someone when I use an AR as I don't play an offensive role. A higher sensitivity on the SMG's as I always play offensively with them, rushing bomb sites/flags ect, and the higher turning rate is pro for those moments where you just turn on a guy and pop him in the head.
Other comments - Those are my public sensitivities of course, I use 4 and 7 when out of public games.