Actually its not so weak. As mentioned earlier, you steal more depends on the amount of damage that you took in a single hit and its usually ore effective at higher levels. I just noticed it that a mage standing within melee range and casting a -aga spell on me and i survived, then it proc and i got tons of hp back.
it seems weak probably because the attack that proc it didn't do alot. If an attack hits you for 80% of your health and you survived while this procs, then you are healed alot. However, if you are hit several times before and then the next attack was like a melee attack that hits for 30 damage, then it will be a little amount.
i prefer counter atk for gaff, its better in arena