cracker - oneways work best on sweeping turns where you can keep on some power (not saying sweepers. . .just turns with some sweep) so that the front literally pulls the car through.
They don't work as well on tight tracks where you have to slow down a lot - you end up having to swing wide initially (which opens you up for T-bones by people with brakes!) and since you can't brake, you have to kinda glide your speed down (or find some other way to shed speed)
It can still be faster, but, as in a conversation with TPhalen this weekend, it is very hard to be as consistent on a tight track with a oneway as it is with a diff.