Teach me when to use Onslaught.

[Syrus_Draco]

It's one of the best-designed gap closers in the game, if not the best.

I'd disagree. So many other melee gap closers do not consume any resources. Look at DRK's Plunge. Zero resource and its 200 potency, where at Onslaught costs BG and is a small 100 potency. Far from being the best desgined gap closer.

That said, I am not denying that it is useful in many scenarios.