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.