Some of you are wrong. The damage shield it creates lasts only 8 seconds, therefore if the raid takes no damage within said 8 second the proc is wasted.
The mace seems like a nice bonus for raid healers, but it also seems too unreliable and not very legendary. Not in the same vain of Thunderfury or Warglaives.
My thoughts is the proc seems best used on a Paladin and the worst possible use of it is on a Disc Priest. Every other class lies somewhere between. The tank healing Paladin will always put up the biggest absorb-shields with it and the tank will always eat damage in the 8 second window.
The Disc Priest unfortunately tends to be lowest on effective healing done, and therefore will put up the weakest shields. Further antagonizing Disc, an enormous part of their "healing" is done via mitigation from Power Word: Shield / Divine Aegis (and in theory Renewed Hope) which do not/will not contribute to the shields.
I am seriously disappointed. On the otherhand, as far as I can tell there are no ilvl 239 healing 1handers, so the mace is a sound investment on it's own. It's just not very legendary. The absorb shield does seem incredibly powerful in the hands of a Holy Priest though. Being able to shield up the raid using Circle of Healing and Prayer of Healing (with Serendipity). And with a lucky proc and good timing, Divine Hymn should be insane with the mace.
Honestly. I would of been completely happy if they had just ripped off the effect from
[Crystal Spire of Karabor]. And either update the proc to be number more consistent with heals thrown out now; or have the proc be a 15% increase in the healing done.