(WIP)
I must confess, when I first saw Stonemason, I wasn't really impressed. I thought that, like with Masterpiece, this would only ever be worth overpaying for, and then it would become a dead card in your deck. But my opinion of it has changed for the better, and while it's certainly not a power card, it's certainly not a bad one, either.
What does it do?When played, Stonemason trashes a card from your hand, then gives you two new cards each independently costing less than the trashed card. When bought, Stonemason comes with two Action cards each independently costing exactly however much you overpaid (if you overpaid for it). Let's look at each of these separately.
Trashing is generally a good idea - it's telling that even a Gardens deck, which lives on bloat, can benefit from some Chapel-ing. However, good trashers either trash extensively (like Chapel), or give you a good benefit with the trashing (like Death Cart), or a choice between trashing and benefit (like Steward). Trashers like Develop (which Donald X has stated spent some time with Stonemason's overpay) are generally not thought of much because Develop is slow, trashing one thing at a time, and giving a benefit that is highly dependent on the kingdom. To this end, we can roughly categorize trashers into early-game and late-game groups. The "good" trashers tend to be ones you want as soon as possible. Stonemason, were it not paired with its overpay, would be firmly in the late-game category. It's quite a bit more flexible than Develop, as it doesn't have the "exactly" restriction. However, unlike Develop, you generally will not want to trash Estates with Stonemason, as you'll be getting at best two Coppers (barring Poor House in the kingdom). Trashing Coppers is fine, as you gain nothing in return, but again, one at a time trashing for the simple sake of trashing is not "good." Upgrade, another one-at-a-time trasher is good because it is a cantrip. Remodel is good because it can actually improve cards, rather than just vanishing them. But in the late game, Stonemason shines. Stonemasoning a Gold into two Duchies is almost as good as Remodeling a Gold into a Province - you get the same amount of points, just two "useless" cards instead of one, which, if you're not shuffling anytime soon, makes no difference. There may be other cases where you would want to Stonemason a Gold into two engine parts or something similar, but that will be dependent on the Kingdom. Stonemason is also decent as a Curse trasher, as it does not require you to gain something in the Curse's place.
The overpay option gives you a reason to buy Stonemason before the late game. The opportunity cost of buying trashers like Develop or Trade Route is negated here, as he simply tags along as a consolation prize with your engine parts. If you have a Village and Terminal draw card of the same cost in the kingdom, pick up one of each with Stonemason. Then use him to slowly trash Coppers until you're ready to start greening.
The overpay option can also ameliorate a poor opening split; if you open 5/2 on a board with key $3s, like Ambassador, you can use Stonemason to still pick up two of them (and then Ambassador your Stonemason to your opponent).
Bear in mind that when overpaying for Stonemason, you're gaining three cards on a single buy. If all players are using Stonemason to build an engine, expect a quick three-pile ending. You can even use this to your advantage by buying out the Stonemasons and some other cheap piles once you know you're in the lead. Or focus fire on the Stonemason pile - Stonemason something into two more Stonemasons, then overpay on Stonemason to gain two extra Stonemasons. Empty pile!
Also remember that each Stonemason overpay does come with a Stonemason. If you don't have some way to use, sift out or get rid of the Stonemason, you may want to think twice about getting one, overpay regardless, especially since Stonemason can't really do much with another Stonemason (unless you're going for an all out Gardens game).
Are there any cards with particular interest in Stonemason?Yes! Border Village is spectacular with Stonemason - overpay by $6, and you get two Villages and two $5s! Plus this mediocre trasher! It's quite a deal. Then in the end game you can crack open those Border Villages into Duchies! If you manage to pick up some Hunting Grounds, you can turn them into three Duchies. Likewise Feodum can be turned into five Silvers. In a similar vein, any Gold-gainer (Market Square, Tunnel, Soothsayer) goes well with Stonemason, as they give you plenty of fodder for turning into Duchies.
Peddler is an interesting case - its cost makes it likewise good for trashing, but be careful when getting them - if you play more than 3 Actions, you can't overpay to gain them.
Stonemason is great on a Potion board - you can get a lot more out of your Potion each shuffle by using Stonemason to pick up two Alchemists/Familiars/Scrying Pools/Golems. Then, once you're done with the Potion, turn it into two Silvers! Or Estates, depending on how far along into the game you are. Speaking of Potion cards, Vineyard is Stonemason's best friend. Every time you buy Stonemason, your Vineyards go up a point.
Stonemason interacts interestingly with cost-caring trashers like Apprentice and Bishop. Instead of feeding a card to the cost-caring trasher, feed it to the Stonemason, which will give two additional cards of slightly lower cost - in the short term this means a net gain of "cost" for your Bishop or Apprentice to feed on, but in the long run will have diminishing returns.
In a similar vein, Stonemason can be decent with Gardens. Every time you trash with Stonemason, you're adding an extra card to your deck (as long as you're not trashing $0 cards), and every time you buy (and overpay for) one, you're adding three cards. That can snowball quite nicely.
Finally, bear in mind that Stonemason's gain can get around other buy restrictions. Played a Contraband and your opponent named Journeyman? Well, I'll just overpay on Stonemason by $5 and get two of them. Have Coppers in play? Overpay on Stonemason by $6 and get two Grand Markets.
Is there a counter to Stonemason?I'm not sure that's really a valid question. If your opponent is using Stonemason to quickly build an engine, you should probably should have been mirroring that engine (unless your opponent just has no idea what they're doing). If their engine is already up and running and you just noticed Stonemason was on the board, it's probably too late for you. Stonemason, like I said, isn't really a power card when played, and is not an Attack, so there isn't really anything to counter per se. Just get your engine up faster, and use all the usual Attacks to slow down your opponent. If your opponent isn't fielding much money per turn, they won't be able to overpay on Stonemason, so discard and Cursing Attacks will be your friend here.
If you are mirroring, make sure you get good Stonemason overpays first - make sure you get the better of engine part splits. If you're playing a board that's likely to end in 3 empty piles, keep track of pile sizes and of how many Stonemasons are left - if it's a Vineyards game, you don't want your opponent to get the better of the Action card split!
Synergies:*Engines with similar-costed parts
*Boards lacking +Buy
*Border Village
*Gold-gainers
*Potion boards
*Vineyards
*Actions with on-buy restrictions
Anti-synergies:*Big Money
*Better trashing and +Buy
