Like most Permanents, Stick to the Plan is really good. It basically says you start the game with three extra good cards in your hand. What's more, you can choose cards to let you consistently execute specific strategies. Here are some possibilities for our three Guardians (not all of these cards are legal for all of them):
Emergency Cache, Ever Vigilant: These cards are very powerful on turn 1 as they let you set up far faster. Now you can start with one (or both) every game. Ever Vigilant, in particular, is a gamechanger for asset-heavy Guardian decks. Guardians have (except for Zoey) hitherto struggled to pay for all their powerful, expensive assets; with Stick to the Plan I think those struggles are mostly a thing of the past.
Extra Ammunition: Starting with this is a huge boon to any investigator who's running a Lightning Gun or Shotgun deck, since it basically doubles the power of those (already very powerful) cards. Goes well with the two options above, since Prepared for the Worst will help you find your gun and an economy card will help you afford it. Then you can load it up. Zoey could even take Contraband for even more ammo, theoretically. You can fetch the upcoming "Eat lead!" to complement your Extra Ammunition, too, if you want.
Shortcut: The upgraded Shortcut just dominates certain maps, especially those with a central nexus location you have to travel through repeatedly. With this Roland can always get the upgraded Shortcut down on turn 1.
Elusive, Dynamite Blast: These are really powerful but slightly situational events, and they're pretty nice things to have in your back pocket. If you have foreknowledge of the scenario, you might know in advance if one of them is going to be needed.
6 XP is a lot, but I think this card offers so much power and consistency to a deck that most Guardians will want to pick it up sooner rather than later.