This looks like an excellent Guardian upgrade. It's kind of similar to Magnifying Glass, in that it costs 1 resource, is Fast, and gives you +1 to your staple action for the rest of the game. That's great. Assuming you play this pretty early, it's going to way outshine what a skill card could accomplish.

The best use of this card, of course, is going to be sticking it on a Machete. That's the best level 0 Guardian weapon, and it works particularly well with Reliable since you can use Machete all game long, provided that the encounter deck doesn't discard it (something you should have been taking precautions against anyway). A multiplayer Guardian who specializes in monster killing (that is to say, most Guardians) can easily use a single Machete 10 times or more over the course of a scenario. That's a lot of value for 1 XP. If you don't draw your Machete, Reliable is also pretty good with the .32 Colt, or, for Skids and Leo, Switchblade II.

The doubled to-hit bonus is much appreciated on high levels. On Expert, you really want to consistently hit +4 (or sometimes +5) on your Combat checks. Prior to this card, that was quite difficult for most Guardians to reach. Now, it should be much more easily achievable. For Skids, who needs to overcome a mediocre base 3 Combat, this is an even bigger boon.

The fact that Reliable gives you a bonus to all skills, instead of just whatever you're testing, is intriguing. As the card pool currently stands, I don't think this really matters for any card except Lockpicks. But it's something to keep in mind as new assets come out.

I noticed that in the comments section, a few people have asked/speculated whether 2 copies of Reliable can be attached to one item (ex: a Weapon + Reliable + Reliable). I figured I would ask the question in the Reviews section, so that it's clear for future readers.

Can you double-stack Reliable on one item?

Does this card work with Lola Hayes, when she's not in guardian? This cards ability has no so i think it's a constant effect and works.

If it works, could be a combo with Otherworldly Compass, Lantern or Lockpicks.

This card approaches taboo level for the cyclopean hammer. Being able to have +4 (each copy gives +1 to each stat, i.e. boosts both might and will, since hammer uses both) to your combined stat of will and might is ridiculous and will even have 'unbuffed' guardians testing at a 10+. I pretty much consider this a standard upgrade anytime you're running a hammer build.

When do you start and stop having +1 to each of your skills when using this? Can you use this with a fast event to get the bonus when doing something unrelated to the action on the item this is attached to?

For example, let's say you attach this to machete. You activate your machete to take the fight action, and that causes a combat skill test. During the player window after committing skill cards to the skill test in the 8 steps of a skill test, you attempt to Expose Weakness the enemy you're fighting. Do you have +1 from reliable during your expose weakness test?

Like, clearly you have the bonus during step 5 of the fight test on the machete, but I honestly do feel like you are getting the +1 starting before step 1 and ending after step 8 because the entirety of the test is part of the resolution of the action taken on machete.

There is a neat little trick combining this card with Eye of the Djinn to get a based skill test +6 (two copies to get +7) once per term using Eye of the Djinn's ability. The three investigators who can make use of this combo Tony Morgan, "Skids" O'Toole, Leo Anderson will all appreciate it.

-- Update: after checking with Fantasy Flight AHC about the ruling. The combo unfortunately does not work.

This is what Fantasy Flight wrote:

• All of the ability text on Eye of the Djinn would resolve before drawing skill tokens for the test. Eye of the Djinn will set up a “lasting effect” (base skill of 5 for the test) and two “delayed effects” (if you draw a Bless or Curse token, do X) that can still affect the game, but the card itself has resolved.
• Consequently, Reliable will only grant you +1 to your skills while the asset it’s attached to is resolving a triggered ability. The timing window for this modification ends when the asset it’s attached to is finished resolving—in this case, Eye of the Djinn will have finished resolving once you begin drawing tokens for the skill test, and you would revert back to having a base skill value of 5 until the end of the test.
• (This is different than if Reliable were attached to an asset with an action triggered ability, such as a weapon with a Fight action.)