When I used it the first time I got 167 HP back... so either it's random, or it could be 10% of Max HP (Since I'm a Mnk it would be around that) Only used it once so far though, don't want to waste the charges.
It's kinda handy if you have spare money and need a Justaucorps anyway. I use one for HNMs and stuff, an extra cure can be handy when I'm getting hit with lots of AoEs. But it's a kinda small amount of HP, so maybe not worth the money...
Although this item has quite few charges does anyone know how to recharge the enchantment. I tried trading to Gate gaurds Jeuno and Home Town but to no avail seems to be a waste of money if I am not able to recharge it.Also unable to put it up for auction in a partialy used state.