I stayed away from this AA since it was reported as broken - conflict with Gift of Mana, which I had purchased.

Now the issue appears resolved, can anyone provide some feedback on how useful this AA is?

Considering the level of content most of us are in by the time we get to purchasing this series of AA, it seems marginally useful at best. Since it doesn't cost mana or a spell slot or buff slot, it's a decent AA, but certainly for me anyway not very high on my list of "must have". It's hard to tell how useful it is really, as you don't get any notice that it's triggered even. But do the math and figure it out from there, compared to the dps put out by the mobs you are fighting. It's not very impressive, more like a perk.

My wife and I tested this over the weekend. She plays a Druid and I play a shaman. We set up an audio trigger to fire off when abundant healing went off. As soon as it went off, I Cannibalized. It looks like abundant healing is a 365 hp per tick HoT that last for 30 sec. I am not sure what or if any focus works on this AA. My wife has 30% healing focus if that matters. She has level 4 of this AA.
From what I have read, at level 5 the AA should fire off on 9% of the heals, but we did not verify any percentage. I could see this being a handy thing when it goes off, providing a little boost in healing for 30 sec. My wife seems to be put into a role of healing more and more often, so she decided to max out all healing AAs, this being the last AA she has to finish off.

My wife and I tested this over the weekend. She plays a Druid and I play a shaman. We set up an audio trigger to fire off when abundant healing went off. As soon as it went off, I Cannibalized. It looks like abundant healing is a 365 hp per tick HoT that last for 30 sec. I am not sure what or if any focus works on this AA. My wife has 30% healing focus if that matters. She has level 4 of this AA.
From what I have read, at level 5 the AA should fire off on 9% of the heals, but we did not verify any percentage. I could see this being a handy thing when it goes off, providing a little boost in healing for 30 sec. My wife seems to be put into a role of healing more and more often, so she decided to max out all healing AAs, this being the last AA she has to finish off.

I can't remember the specifics, it was either 9% firing with 5 levels in GoM and 10% without it or GoM fired 9% with 5 levels in AH and 10% without it.

Either way, 1% is so marginal, you'd really have to be a statistic nazi not to buy all levels for that reason alone.

The issue was fixed from what I heard. At max ranks, GoM procs 10%, AH procs 9%. If GoM procs, then AH can't proc.

I went ahead and re-purchased these. Now have GoM-5 and AH-5. Although I cannot tell you about the AH procs, I can tell you that it doesn't interfere with GoM procs any longer.

I'm not getting AH unless I'm completely done with AA's.

You do get a notice when it goes off. I forget exactly what it says but you do get it.

It heals too little to really count but I bought it as soon as it was fixed anyway, just to have all the healing I could possibly get.

Definitely needs to heal more to be worth it.

I have this maxed on my druid and my cleric and hardly notice a difference on either.

I use my druid to tank quite a bit in the nest while healing myself with my cleric and it is very rare that I notice it firing. Even when it does, I cant really tell the difference. Of course the Drakes hit pretty hard so a 365 pt heal per tick etc is negligible.

I am sure there are plenty of situations where is serves its purpose but if I was trying to decide where to spend AAs now, I would certainly put of AH until just before Packrat.