At a race with this lens right now and so far not at all impressed with its auto focus in low light, during the day I love this lens but, last night trying to take shots at the track even with flash I couldn't for the life of me get this lens to focus on the cars.
I don't understand what the flash have anything to do with the AF? Normally the flash is fired after the AF already be acquired. Normally every lens and body have a limit on the AF. There is a trick that help acquired good AF, not always works but it help. Say, instead of focusing on the eyes or face when shooting in dimmed environment. Sometime I focus on the feet or accessories on the body. While yes it's not the same as focus on the eyes but at least that normally get the focus in a good ball park and better than focus on something totally far off or not able to AF at all.
PS: I am thinking about getting this lens also, but maybe waiting for the next round of rebate.