Mint is... interesting. Quite powerful, but difficult to buy correctly - ideally, you want to already have some silver and gold and then buy this for all copper to get rid of them.
Initial Observations: Mint: anders summed it up quite well. try to buy it using mostly coppers, then lay it to duplicate more valuable treasures.
Mountebank: Interesting. Most of the time discarding a curse won't harm you. A powerful attack due to opponents gaining a curse and a copper, but gets less powerful as people get more curses since they can block it. Less powerful also if your opponent(s) want coppers (Counting House, Coppersmith, Bank, Gardening). Doesn't really work against gardening players.
Rabble: Harsh, because useless Vicotry point cards stay on top of your deck. I think it's best compared to Smithy (cost 4,+ 3 cards), for one more coin you get an attack effect, but the attack doesn't seem that amazing.
Buy Mint/Copper -> then silver -> silver -> Gold -> Gold -> Mint -> Gold -> Adventure -> Province -> Province -> Province -> Province -> Province -> Province -> Province -> Province
Mint's interesting because it's not primarily an engine card, it's an opportunity card. It goes amazing with particular hands in particular situation, but not most. I'm intrigued to see if those situations actually occur often enough to justify the card on most boards.
And rabble actually just looks unreal. I'm a huge fan of drawing three cards in almost any situation, even without the action, and the upside is tremendous.
Mint makes me wonder about something... how, specifically, does the buy phase work? Do you have to play all treasures you plan on playing during the buy phase, then buying your cards? Or, can you play a couple treasures, buy something, play a couple more treasures, maybe buy something else (if you have an extra buy), although with the new fancy treasures, you may want to play treasures even if you aren't buying anything. So when exactly can you gain a card during the buy phase? Only after all treasures you plan on playing are played, or whenever you want?
Already answered in the Prosperity rules at RGG. Play all treasures you wish to play, one at a time. Then make all buys. You don't have to play all your treasures, but once you start buying, you can't play more treasures.
Thanks. That makes Mint slightly worse than it would be otherwise, but I guess it will usually only matter if you were going to buy Mint and another card.