Re: your comments above. If I ever use the 2x, I switch to manual focus because, as you quite rightly pointed out, by the time it's focused using AF, the race would be finished!
As I said though, I only ever use it in 'extreme' circumstances (I think I've used it twice in the 12 months I've owned it) so I'm not unduly worried by it's relative lack of performance.
It does stop down by 2 which makes low light photography a no-no, unless I go for a high ISO. Unfortunately that means a nasty amount of noise on my D70. Might have a look at a 1.4x though if you only lose 1 stop. May be a better option.