This is by far the most interesting of the new Synergy cards. There are a number of ways you could view this card:

• A source of extra actions. At a cost of 2 resources, a play action, and the card itself, you need to get all five charges to unambiguously have a net positive (in terms of "click economy"), but if you have extra resources or actions anyway -- unlikely but possible for most mystics -- this card can be a solid way to do more things over time. It also lets you bank actions for the future.
• A source of actions. With multiple investigators, it's not uncommon to find situations where you want gator A to do some things before gator B, and some things afterward. This gives you a chance to have your cake and eat it too by letting you interleave some of their actions. The actions from this card don't provoke attacks of opportunity either, so you could also use it to, for example, move off a location that makes it harder to fight monsters before laying into them. EDIT: My understanding of the rules was incorrect. Since you are instructed to "Take" the action, the action you take will provoke AoOs. Compare e.g. Ursula's ability.
• A way to substitute for other attributes. Mystics generally like pumping their willpower anyway, so getting a chance to substitute it is always a good thing. It reminds me the most of Sword Cane in that regard; it's a handy way to evade with , or fight a 1-health monster. You could also use it to investigate, though this forfeits the action compression of spells like Rite of Seeking and secondary effects of spells like Sixth Sense. It's a good way to save a charge if there's only one clue left, or if you really don't want to worry about drawing a symbol token.

When you consider that you'll often get several of these benefits simultaneously, this card seems really potent. So what are the downsides?

• Obviously, this card requires you to run cards from several different classes (ideally, all of them) in your deck, unless you plan to manipulate charges some other way. Furthermore...
• You need to not only have multiclass cards in your deck, you need to have them on the field. This means that (1) skills and (most) events don't count, so you need to run assets, which might start to tax your slots, and (2) the order you play your cards matters a lot. If you draw this before drawing any of your multiclass cards, it's likely to just take up space in your hand for much of the game.
• It takes up an Arcane slot. This is a really big ask, since most mystics want to fill their arcane slots with some combination of an investigate spell and a fight/evade spell. Fortunately, there are many options for getting more slots, but I don't think I'd ever consider running this card without at least one of them.
• It's not a Spell. This is a surprisingly big deal, because it means it lacks synergy with most of the mystic suite. You can't reduce its cost with Robes of Endless Night or Shining Trapezohedron, and at 2 resources it's an inefficient use of Uncage the Soul. Most recharge options don't work on it (there are a few, like the newly-released Winds of Power, or Eldritch Sophist if you have access). It also can't be tutored by Arcane Initiate.
• You can only take basic actions with it. The EOTE expansion contains an FAQ (not on arkhamdb yet, to my knowledge) which defines basic actions to be those without a bold action designator on a card. So you can't use this to trigger e.g. Rite of Seeking, but you could use it for a normal investigation. This is only a minor downside in my opinion, since you take dozens of basic actions per game, so you'll have plenty of opportunities to use this.

In summary, this card has the potential to be quite useful in the right circumstances, but requires a fair bit of support (multiclass cards and extra arcane slots) to see the full effect. It's certainly worth a look for anyone who's planning to run multiclass decks anyway -- in fact, it might even be better for off-class mystics, since their arcane slots are usually less contested.

There's one mystic who really loves this card, though, and that's Akachi Onyele. She already had access to strong out-of-class cards even before Edge of the Earth dropped and she works great with "bling archmage" builds (The Hierophant • V plus Relic Hunter plus Sign Magick) which take care of the slot problems. Most importantly, though, is the fact that this card uses charges. As a result, not only does she get an additional charge each time she plays it (essentially refunding the play action), she can keep playing it over and over with her signature. Not only does this mean she can just keep getting extra actions than usual, but she can play it early (before her multiclass cards are out) without mourning the lost charges. Other mystics might be a bit skeptical, but I'm probably always going to consider this card when building an Akachi deck in the future.

Handcuffs is a good comparison.

Pros:

• Takes no actions.
• Works on enemies other than Humanoids.
• Testless.

Cons:

• Doesn't turn off Mysterious Chanting or other similar cards.
• You pay for it in health and/or sanity. Some enemies are worse than others in this regard.
• Costs 3 more XP

I think the summary would be that Handcuffs is a fairly specific tech card against scenarios with a lot of doom enemies that can be useful in other situations, while Fend Off is a bit more generically good. I'm personally pretty excited for this one. All or Nothing Skids can also take it!

Cons:

• Some cards that you would want 2, like permanents: Charisma, Another Day, Another Dollar.

Pros:

• Parallel Skids and his 20 card deck.
• Exceptional rogues as mentioned earlier.
• It doesn't limit bound cards (Hallowed Mirror, Occult Lexicon)

Other:

• Interesting that using In the Thick of It+Versatile+Underworld Support nets you 2 traumas and nothing else.
• Rogue - Survivors can start with 15 card in deck, but as all card is limited to 1 instance and 10 are discarded don't know if there is a good combo for this. (Underworld Support+Short Supply)

This card card can cancel Crypt Chill and Corrosion so if you're an asset heavy guardian or survivor, you know how easily these cards ruin your day. Also works for snake bite and grasping hands which are nasty little agility tests that can be pretty cruel.

Does this work with drawing thin? Is this still considered “initiating a skill test”? If so this’ll be even more amazing with Calvin, because then you can just boost the difficulty of the test to draw cards or gain resources, while still knowing that you will win. Additional plus of using this in conjunction with Counterpunch to just annihilate a monster in a single action.