Duality

[Ryaz]

Been leveling my ninja and I'm just about to pick up Duality, but I'm bit confused on how it works exactly. After some quick searches on the net, I've seen mixed opinions.

I know it only applies to weapon skills. And judging from the tooltip, it applies the extra combo bonus on just the first attack, meaning that the extra damage amounts to 100 potency on most skills and 150 for Spinning Edge, either no chance of a crit? If so, the ability seems a bit lackluster, especially given its somewhat long cooldown. How is it in practice?