When I was in Los Angeles for Thanksgiving, I began noticing the hand gestures that accompanied numbers. When people said "six", they often punctuated it by holding out their hand with the thumb and pinky extended, palm towards the speaker. That's because they were using Chinese number gestures. (It so happens that the gesture for "six" is the same in both the Chinese and Taiwanese systems.)
What was particularly amusing was that when I asked them about it later, they had no recollection that they had done it and didn't even notice that I was doing it.
"I noticed you said 'six'," as I make the 'six' gesture.
"Right, it weighed six pounds."
"But you did this," and I make the gesture again.
"I did what?"
"This. The hand."
"Oh, the thing with the hand. Right, that means six."
The use of the hand gesture was unconscious and automatic.
As for me, I use the Korean system of Chisenbop which I taught myself in sixth grade from a book. To me, it's much more logical than the Chinese, Taiwanese, or ASL versions, all of whom I had taught myself at one point or another but now can recall only with some effort. In particular, Chisenbop lends itself much more easily to doing computations rather than merely conveying a value. For example, adding nine is "down one on left, up one on right". You can do computations on your hand like an abacus, paying no attention to the actual value but merely manipulating your fingers mechanically, and then when you're done, you look down at your hand to see what the result is.