I don't think it can be overstated how powerful this card will be for either both front options for Agnes (with regular back). She will be able to test most of her Willpower tests at an effective base of 7.

Regular Agnes: If you can take horror regularly enough (Ward of Protection, Forbidden Knowledge etc), she can get multiple uses a turn.

Parallel Agnes: Due to being able to choose when to deal damage to herself, I think Parallel Agnes is even stronger. Assuming she has the soak available (which she should, it's an Agnes deck), she should be able to improve every test she has to make.

Another combo I thought of that Benwobbles didn't also mention, you can also use Smoking Pipe/Painkillers as batteries for this.

I can imagine this being a solid card for Calvin, or in conjunction with Spirit of Humanity. Throw in all the Survivor self-healing soaks (Peter, Jessica, Precious Memento) and you've got some sustainable combinations.

As of writing we have only seen one other mask with its boost and replenish condition (Cat mask, mystic, boosts Willpower/Combat, and replenishes both offerings on doom being placed on a card that doesn't have any).

I think Sparrow Mask will likely be the best, or most universally useful. Willpower and Agility are not the 'do something' stats, when you need to pass a test for these it's often critical for an encounter/scenario card. +2 can push a coin toss to very likely to succeed, and I think this will have wide play for investigators who need it. Bob (especially), Darrell, Rita and Daniella can probably all use this to shore up some of their weaker side (HP for Darrell, Sanity for Rita and Daniella) by shoring up their defenses, while Bob can hand out masks to others.

With respect to the masks in general, they are all (likely) level 0 so they can be used by the Dunwich investigators which also seems strong for Pete (boosting Duke's tests if there is a combat/intellect one).

Edit: How wrong I was, these masks are all insane. Getting primary stat boosts multiple times per scenario is incredibly good, and at only 1 resource even getting the effect twice is fine. Guardians get combat, seekers get Intellect, rogues get agility AND intellect. These are possibly what the talent cards in the base set should have been.