"You owe me one!" is a unique action economy event whose inefficiency is made up for by the possibility of playing a card your investigator might never have otherwise had access to. It's kind of like a mini-Teamwork, but where Teamwork costs you a card, "You owe me one!" replaces both itself and the card you "borrowed". This ends up being action economy because, as an example, had your partner played their Emergency Cache (0) on its own, they'd be down a card and up three resources, but with "You owe me one!" they get that card back, and someone on the team still gets the three resources. It's almost like playing Emergency Cache (2), just with the resources going to a different player.

If this action economy was all it did, "You owe me one!" would be pretty boring. A fast zero-cost event that draws you and another player a card is certainly positive value (and this is comparable to the drawing cards effect of "You owe me one!"), but it is not a particularly powerful effect. Like Teamwork though, "You owe me one!" can allow for all sorts of crazy combos when you play things outside your cardpool. I'm actually not going to delve too deep into the possibilities here as they're as broad as your collection, and many have been mentioned in other reviews (including for Teamwork, so do look at those too if that's what you're after). What I am going to do is try to show briefly why "You owe me one!" is much better than Teamwork for a typical investigator deck, and also how to best take advantage of it and make it shine.

While Teamwork lets you exchange any number of Item and Ally assets and even trade resources, it has always been held back by its cost. "You owe me one!" only lets you swap a single card, but the meager action economy also offered makes a huge difference to the card actually generating an advantage for the team. Teamwork has to enable giving or receiving something really momentus to justify burning an action and a card, whereas you can play "You owe me one!" and be satisfied with even a mediocre trade. Additionally, "You owe me one!" allows you to play any asset or even an event from another player's hand, so can trade some things that Teamwork cannot, and I think this is generally much stronger than resource re-allocation. The cherry on the cake is that, bizarrely, you needn't even be at the same location! Luke Robinson, eat your heart out!

One big downside to "You owe me one!" being used to set something crazy up is that investigator hands are hidden zones, and the rules prohibit explicitly naming or revealing cards in your hand to other players. This is a fairly soft and fuzzy rule so I'm sure if you were determined enough you could work around it, but even sticking to the spirit of it very closely there are still some ways to know what is in another investigator's hand.

Firstly, Norman Withers. Vengeful Hound aside, Norman reveals every card he draws, and if you pay attention you can be sure what kinds of favors are available. Secondly, cards that add bonded cards to hand. If your partner just played their Occult Lexicon, you know they will be holding Blood-Rite. Do note though that, if you play a card like Occult Lexicon with "You owe me one!", your bounded cards don't include Blood Rite, so the book itself is useless to you (as are many other cards with bonds, such as Summoned Hound, which you cannot play with "You owe me one!" because you cannot pay the additional cost). Thirdly, cards that move other cards from non-hidden zones to the hand will also guarantee you targets; Calling in Favors, Sleight of Hand, Resourceful, and Scrounge for Supplies can all tell you a card in another investigator's hand without breaking or bending the communication rule. Searching hidden zones for certain cards can also give partial information; if your partner just played Prepared for the Worst and drew something, you know they have a weapon you could "borrow"!

There may also be some other edge cases where you can reliably infer a card in your partner's hand. One example I am aware of: if your partner draws a second card with Mr. "Rook", it must be a weakness (they may even announce so). If they don't then resolve a Revelation effect and just add the card to their hand, it may be their copy of The Tower or Dark Pact or Carcosa campaign spoiler, and perhaps you'll then want to play "You owe me one!" to do them a favor and play that weakness for them! "You owe me one!" does not allow you to play weakness cards, so my particular example here is moot. Thank you user SGPrometheus for the correction!

With all those options available, it is entirely possible to co-ordinate your deck with a teammate to make it likely that you know when there is something good to "borrow". Even if you don't do that though, it should be pretty rare for "You owe me one!" to miss entirely unless your partner's deck is all skills and unique allies. Just play it while your slots are open and they have a healthy handsize. You can however also build your own deck to take even greater advantage of "You owe me one!".

Rogues tend to generate lots of extra resources - particularly Preston Fairmont and Jenny Barnes - and you may be able to take and play a card that another investigator was struggling to pool the resources to play. Building your deck to be more generalised also makes you likely to be able to take advantage of whatever you find, rather than being a monster hunter stuck with a Magnifying Glass. Finally, consider the value of combining "You owe me one!" with Crystallizer of Dreams. I already discussed how "You owe me one!" is a kind of frictionless action economy, replacing itself and the card you play with it, which is absolutely perfect for the Crystallizer to grant you effectively "free" skill test icons for the future. Even better if you play another investigator's event that you can also attach to the Crystallizer!

Seems like quite an interesting card with a lot of interactions and not totally clear rules for me, which I hope to clarify here. First of all, it's a Roguish version of Teamwork, but with some advantages. Disadvantages are also present, but they seem pretty minor to me. Let's talk about obvious advantages:

1. An awesome card in terms of action economy. 0-lvl, 0 cost. You trade one action, which is normally used to play one of your card, to play a card of your mate and to get two cards for the team. Unlike above mentioned Teamwork, it also gives something (two additional cards), not only provides distribution of already existed goodies among players.
2. Rogues are the richest faction, so it allows to save some resources for poorer mates, while providing the same benefit for the team. E.g. Guardian's Agency Backup: it may totally devastate a guardian, yet can relatively easily be played by many Rogues.
3. You can support your character's archetype with a card you normally cannot include in your deck. I haven't thought much about it, but Dark Horse for Leo Anderson or some cool weapon for Finn crossed my mind. And now regarding not very clear moments.
4. "To play a card under you control". Obviously, you cannot play a skill card, and it is clear with assets. But what does it mean to play an event card under your control? Just to apply its effect on yourself instead of the owner? It seems like this card allows to play events as well, doesn't it?
5. Attacks of opportunity. I read it as even though you're gonna play a card from your mate's hand which doesn't trigger an attack of opportunity (e.g. Dynamite Blast (2), you will get it because the initial event does provoke it and you apply an attack of opportunity before applying the effect of the card. If you play a card that also provokes an attack of opportunity, it will get you hit only once, as I understand, because you are doing one action (and it is the action that provoke attacks of opportunity) with several effects. There was also a question in comments whether you need to pay resource costs of the card you play. I decided to add the clarification to the review, since a friend of mine also asked the same: yes, you do.Tthe rules specifically say: "to play a card, an investigator must pay the card's resource cost and meet any applicable play restrictions and conditions." Sometimes cards allow to avoid paying the costs, then the wording "put into play" is used instead of simply "to play". Here are some examples: Sleight of Hand, Flare

The most fun "Gambit" card so far I have played. The goal is to deduce what cards this player is hogging before you see them and having enough resource for that card you are thinking of, and for it to actually be useful right now. But unlike chaos token probability-based Gambit, you can master this one by knowing your friend's deck and their natural progression and improve your chance.

For example if the scenario is already passed by for a while, I can guess that my fully-setup Mystic player may have a redundant spell asset on hand which they'd rather exchange for other cards. If someone is fighting for long time with more and more cards on hand, they probably have Asset or Event that triggers AoO and is unable to make use of while fighting while the Skill card that would be a dud for this are mostly used up in the fight. (Check discard pile to confirm) The Guardian player might be holding Dynamite Blast by now because half their deck already passed and there is none in the discard pile and they also have chronically low resource. There is higher chance to find an Ally on one's hand if they don't have Charisma and also have an Ally in play already. (Should not be too difficult to remember redundancy of allies and how many lacks the Unique star in everyone's deck.) This way you can find some quick soak and also give them a new card that is more useful for them.

Discard pile is likely the biggest source of open information so take note of how many dud cards like Skill and reactive Event like Ward of Protection are in the targeted player's discard pile and you can find more hits. You can also just stare at who loves reviving things from discard pile or take card from play area back to hand.