Giving up a grenade for a grapple that goes on Cooldown seems like a hindrance instead of the fun it implies.
Either make the Grapple infinite or make the Cooldown very minimal. It was advertised that you would be swinging around Destiny with the Grapple and speed hitherto unseen.
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What's the most hilarious to me is that there is an internal cooldown EVEN on hunter's two charges!!! W T F?!?! You grapple once, then get a cooldown!!! WHaT?!?!