Voidbringer Conjurer Gloves

For the Voidbringer Gloves, I was wondering about this mod:

80% increased Mana Cost of Skills

Does this apply to all my skills, including gems on different gear pieces or just on the gems in the gloves? If it's global, doesn't the 80% mana cost increase wreck most builds? Am I overestimating the impact?

It applies globally to all skills you cast, but not to auras as they are mana reservation.

I've used them with my fireball crit witch back in anarchy, if your damage is high enough you can sustain the increased mana cost with a mana leech gem.