The quadra is really perfect for action sports as it is the smallest flash with this kind of power, and you can get with the A heads a flash duration of 1/6000S !!
But ! you need to know that you wont be abble to do high speed sync as you can do with your SB900.
You will shoot in manual mode only at max sync of 1/200s but with geater power !! 🙂
I will do a full review soon of the Quadras but you can already find a lot of cool video on Youtube.