Wow, this is really interesting. I feel like I have no good sense for how often I'd want each of these.
Guide -- Obviously this is going to be good in a game with hand size reducers. But how often will I want this without those attacks on the board? It would definitely add reliability to an engine. If I draw a dud hand, I can just skip over it to my stronger cards. Will knowing that you have that ability change the way you build engines?
Duplicate -- Kind of like a Smugglers for yourself. This one's a little trickier than Guide becuase it's not a cantrip, so if you want to do any cool tricks with gaining and playing during the same turn, you're not only going to have to play this early on your turn, you're also going to need to have enough +actions early on to use this and still have the rest of your engine fire.
Coin of the Realm -- This also adds engine reliability. If I draw terminals together, I just call this back and can play both terminals. In one sense it compares to Masterpiece since its on-play effect is only a Copper, but the bonus is what you care about. In another, it's totally different than Masterpiece, since it doesn't spend most of its time siting around in your deck clogging it up.
Overall, reserves are a really cool idea. I can't wait to use them. I feel like they're going to change the way I think about Dominion.