I don't know if this has already been brought to your attention. I play on the Xbox 360, usually as a Hunter. While playing as the Warlock class, leveling up Voidwalker, I got Surge. Surge is great! But, it's also not great when it runs out. Anyone that uses it will tell you that. But, what they might not tell you is that if you're sprinting as Surge's timer runs out your speed is reduced to walking speed while still sprinting. I can't say for sure but I also think it resets you to the lowest amount of Agility if Surge runs out while you're sprinting. I need to level up some more and get some more Agility to test that.