When I do spec into Atonement, I'm pretty much taking the points for evangelism stacks and the AA usage. If I'm already casting Smite for those, I may as well take 'free heals' too. The healing increase is pretty sweet and I like to use it before using a big heal like Divine Hymn for maximum healing during a damage intensive phase of the fight.
I do agree that it's great for 5mans. While I find it pretty great for shadow, it could use some tweaking for disc/holy, specifically because with Shadow it's stacking off a spell that's always in your rotation. Could you imagine if it proc'd off Prayer of Healing or Greater Heal? I guess we'd hear too many complaints about disc being overpowered then ;]
Back to the OP, while Grace procs would be nice, it's pretty sweet to have Divine Aegis proc'ing off the spell too, despite being 'random'. Healing a low-health target is okay by me All in all, it's yet another heal to add to the book and can certainly be helpful if used correctly with a faster cast time and more effective heal than Heal itself.
Edit: The % mana back is pretty low, especially compared to shadow's 5% return. Although, since I'm using the spell to stack AA, it's better than nothing (though 5% would be pretty sweet >.<)