Thoughts, in reverse:
Coin of the Realm: I can hardly imagine the game where winning the split is important. The card is real bad to have in your deck (I bought a copper) for the benefit of being on-call-village-but-I-have-to-put-this-back-in-my-deck. Hm. Building an engine where this is the only village seems like it is asking for trouble (sure, there's the rare case where everything else is perfect). I expect this is mostly going to be something you pick up one or maaybe two of on a spare buy/gain, so that you get a touch of extra reliability. I will laugh heartily when someone (probably me) is missing the $1 from this and it costs them.
Duplicate: Gut reaction was that this seemed pretty weak. But thought a little, and man, it just has to be pretty strong. Not a world-beater or anything, as it's still situational. But a very nice role-player. I mean, once I started thinking about it as a Workshop, it seemed a lot better. Just, you always have the option of doubling that buy of yours from the turn. Plus random upside of being able to Island it out of your deck. The bigger thing, of course, is that you can get 5s and 6s. That is just huge. I am going to guess that turn 3-4 is a very normal time to pick this up. You probably want to think about not opening this if there aren't good 3s you want - certainly not if there aren't good 3s or 4s. tracking shuffles is probably going to be important here, and you will want this earlier on in the shuffle, so you can now whether to take it back now, or risk getting something better next turn. Indeed, all of these Reserve cards seem pretty high skill, as you have significant decisions on if/when to buy as well as big, non-trivial play decisions.
Finally, we get to Guide. Oh Guide. The obvious use, to my mind, is that you can grab this with a spare gain to give your engine lots of extra reliability. A couple of these means you almost never dud out, especially given that you can get them back on the mat fairly easily. Hooray. And it counters discard attacks - especially torturer - there's a plus. Combos with Outpost, that is super nice. My thoughts (and somewhat questions) are about other decks though. It doesn't even seem that bad in Big Money, at all. You can cycle through to your good hands. And if there are just cards you want to play a lot, you can cycle through to them. Stash, hey, that's a thing. More than that though, cards like Rebuild or Junkers - flipping through decks there seems real nice, and 3 seems a real low price. I suspect this is the strongest card we've seen so far here?