I used to play on 5, I even quickscoped on COD4 with 5. When I started getting into competitive quickscoping on MW2, I went up to 10 when I couldn't aim as fast as my opponents. I got used to it with all of my guns fairly quickly. I was unstoppable with it, top 10,000 in legit kills for 6 months straight. I didn't play Black Ops very much. I play on 10 now, still.
I like how I barely have to move the sticks to switch targets in my LOS. If I push the stick all the way over, I'll spin around for a kill on a guy behind me. I play an uptempo game, it feels sluggish if I go to a low sensitivity.