It works well in practice. We've used it for a long time in the playtest, and it's never been an issue.
A lot of people have reported a strong dislike for it, though - we've seen that discussed on this forum, and on other forums.
I note that in my group, they forgot to use it entirely - BUT if the player of the Cleric from my main 4E group had been there, no way in hell she wouldn't have been getting it applied to every possible/practical roll, nor would the player of the Shaman. Those people have buffing/supporting in their souls, and practically every non-combat rolled action would have been interrupted by "I pray for his success" or however one wants to RP it.
I know from other games that that kind of thing can get very tedious very fast.
It's also one of those idiotic spells that CLERICS can get, but BARDS can't, and it usurps the Bard roll.
I mean -
Bard - Can give you +1d8 (avg. 4.5) to a roll maybe 4-5 times a day before L5, maybe 8-15 times after that (and the die gets a bit bigger). You have to seriously think about when to use it, especially out of combat. In combat it slams into your action-economy, hard, too, because it requires a bonus action and must be used pre-emptively.
Cleric - Can give you +1d4 (avg. 2.5) to pretty much EVERY single non-combat roll ever. No thinking about it, no questions beyond "do I have an opportunity to cast it?". By level 5 or so, can easily be casting Bless most combats, giving you +1d4 to ALL the attacks and saves for the whole combat - no thinking about it, no questions, only limit is three targets (if cast with an L1 slot). Only impact on action economy is initial casting time.
Pretty sure the Cleric is a seriously more powerful buffer, because there's hardly ever any question of NOT using Guidance or Bless. Cleric is really stamping Bard toes here.