If your drag racing leave it in D or use M (Sport mode -without touching the paddle shifters). You'll go slower using the paddle shifters (trust me).
There's a slight delay on the shifts when using the paddle shifters and it can be hard to get the shifts just right. The computer does a much much better job of shifting.
The paddle shifters are fun to play with. You can up shift and down shift and have some fun. But the car is faster when the computer does the shifting.