My old Tae Kwon Do instructor used to say that yellow belts--the second-lowest rank--were the worst to spar against. I think he said this because they have just enough skill to know the basic moves, but not near enough skill to know how to control those moves.
Buying a Mint with a 5/2 opening is almost always a "yellow belt" move. Why? Because, congratulations, you now only have two coin left, plus whatever the $2 card was (if you could even get a decent $2 card). And until Hinterlands, there were only two $2 cards that could give you +Coin without consuming an action, certainly neither of them Treasures. But even then, the only Treasures left to mint were Coppers, and why would you want to mint a Copper? What Mint really needed to justify opening with it was some kind of Treasure that, at $2, would be worth its weight in gold. (Ha ha.) Something that maybe wasn't so great on its own, but once you got several of them, were incredibly strong.
Enter Mint's long-lost love: Fool's Gold. Opening Mint/FG is spectacularly powerful for two reasons: One, FG is heavily dependent on deck-thinning. There's just no sense in buying FG if they are spread all over your deck--hence the apt name. You need to repeatedly have at least two of them in your hand to justify their investment. And second, Mint needs to be able to find good Treasures to mint, not mediocre or crappy ones. Mint brilliantly solves both of these problems: it gets rid of the useless Treasures and multiplies the useful ones, and it does so with a high rate of speed and success. It should be no surprise, then, that Mint/FG is, by far,
the best opening with Mint; the only other Mint openings that don't completely suck, not including outlier openings, are Mint/Lighthouse and Mint/Chapel, and there are far better ways to open with those two $2 cards. And it's currently rated as
the fifth-best opener in the entire game, right up there with powerhouses such as Tournament/Ambassador and Witch/Chapel.
This raises the question, of course, of what to do with a Mint/Fool's Gold board when you have to settle for the more common, 4/3 opening. If you're lucky enough to get a five-Copper hand on turns 3 or 4, then by all means, get that Mint. Trashing a Silver and three Coppers is risky but still doable. But remember, it's rarely a good play to sack a Gold, or probably even a Fool's Gold for that matter, just to get a Mint. Remember, this is a $5 card we're talking about.
Another word of caution: Thief (edit: and Pirate Ship). Thief LOVES strong, tightly-concentrated decks. There's no fun in watching all your hard work of building an FG deck be lost to someone who bought what is usually a very crappy card, only to see it actually be a strong attack for once. It's the same reason why you really have to be careful with chapeling when Thief is on the board. (Note, BTW, that FG is immune to Noble Brigand attacks.) But, if your opponent starts thieving, you could just grab a Thief of your own. And because your deck should be a lot thinner than his or hers, yours will attack more, hopefully grabbing back a lost FG or two.
All-in-all, it's a rare combo to have the chance at, because it requires two specific cards on the board AND a 5/2 opening. But if you have the chance to do so, give it a shot. You just might want to try it again!