The Key of Solomon is one of the best cards for the Blurse archetype, to the point where characters who normally wouldn't care about Blessings or Curses should potentially think about running Blurse solely to fuel it.

For Curse investigators, The Key of Solomon is the best economy card in the game that refunds itself on the same turn it's played, with the added bonus of removing 1 Curse token from the bag per turn at Fast speed. As soon the Key of Solomon is in play (preferably with a card like Spirit of Humanity or Gabriel Carillo to consistently fuel it), Curse investigators can give Rogues like Jenny Barnes or Preston Fairmont a serious run for their money in the resource generation department.

For Blessing investigators, The Key of Solomon is the best healing card in the game on par with Hallowed Mirror paired with Soothing Melody. Being able to consistently heal investigators and allies for any combination of 2 damage and/or 2 horror every turn without needing to spend actions is pretty incredible. Unfortunately, it's a bit harder to keep the chaos bag flooded with Blessings than Curses, but the payoff is strong enough for Blessing tech that even investigators like Carolyn Fern and Vincent Lee should seriously consider running at least running Spirit of Humanity, Radiant Smite, and The Key of Solomon for the synergy alone.

And if your table happens to be running any combination of Blessing and Curses, you get the best of both worlds. I'd be shocked if The Key of Solomon wasn't on the next Taboo list, so abuse it while you still can.

Both modes of this card are good, but particularly the Bless consuming mode is just very silly. Healing for two every turn, on whichever investigator is in the worst shape, is game breaking. Suddenly you just don't have to care about random treacheries hitting someone for 2-3, and you can even contemplate letting someone tank a turn of repeated Attacks of Opportunity because they were full to begin with and you can heal them up after.

You don't need to fill the bag, just a few blesses will enable a lot of healing, and cards like Ancestral Token on your fighter will easily power this, or a well timed Keep Faith should be good for at least two uses of healing. You can also mess around with things like Tempt Fate to let you get access to both sides of the book, though note that the book can't be used when the bag is exactly balanced. As soon as someone draws either kind of token, though, your book is back online at the next player window.

4 xp may seem like a lot, but you don't necessarily need two copies, and you don't necessarily need to find the book in your opening hand, or even early in the game. Getting it out on turn 10 when things are starting to get dicey is still very valuable, and if you have the blesses ready to go you can heal for 2 immediately and then again the first time someone has a skill test in the mythos phase. That's plenty to get someone out of danger.