Once you stacked up some resistances, it gives compareable effective hp(maybe still a bit less), but the sustain is actually pretty powerfull since the buffs. If you play a champ that often can drop out of a fight with low hp, it can basically give a second hp-bar for the end of the fight.
It's also strong if you have alot shields in your team, cause damge done to shields does not disable the passive, so if you get shielded for over 5sec, the regen starts to kick in mid combat.
On a champ like Sion with locket and some shielding support, this happens quite often unless you get full focused fire.
All in all, it's a solid pickup as 5th or 6th item for many tanks and some fighters like Wukong.