The name of the game with Contraband is options. Well, use of the buy, too. But mostly options. In, say, a standard big money deck, contraband is pretty bad. Early on, they can pin you off of gold, and later, they pin you off of province, which just makes it very difficult to win. And indeed, in any case where you're really needing one card, it's not going to be good for you. It's also rarely good in multiples, because if one thing doesn't totally block you, two much more likely will - they can pick different things, people. It's weird - contraband tends to be best in engines, because those tend to have the most different cards that can help you, can make use of the buy, and proscribing just isn't as big a deal.
Because of the enormous complexity of choices, the card doesn't simulate so well. So unlike most of my articles, there's no sim data backing this stuff up. But lots of thought and experience.
Before we go any farther, I think it's important to note a couple of tactical items about playing contraband. The first is that you don't have to play it! Many times, in the ending, you need to be buying a particular card - say a province. Well, if you can afford the thing anyway, just don't play the contraband! Second, if you do play the contraband, you pretty much want to play it first, before your other treasures. The less information you give to your opponents, the better, as they can't know what to proscribe nearly so well. This leads to the corollary that Contraband is not so good with the handful of cards that reveal your hand to your opponent. Menagerie might be the biggest exception here, especially when it goes off, but even it can have some weakness. Moreover, it's pretty bad with hunting party, crossroads, etc. - really anything that gives your opponent a good guess at what you've got in your hand. This means that draw-your-whole-deck probably makes playing contraband a bit of a bad idea too.
Playing with Contraband:
The biggest thing is that you need here is, as I said, options. So big things here are nobles, harem, hoard, goons... the de facto anti-contraband strategy early on is to block gold every time, so any time there's something you might well want more (which usually means a 6, as a 5 you would have bought over contraband in the first place) is good. And anything that you want almost as much is also fine. Of course, later on, the main thing to block is province, so alternate VP can help a lot too. The important thing here is that you need to maintain some flexibility. If you're so locked into your duchy/duke strategy, for instance, they cn block duchy all the time early, duke all the time late, and your ability to buy province or gold doesn't help you so much. Of course, even so, you have SOME options here - duchy or duke, as we're assuming you can pull off a good number of one or the other on your non-contraband hands anyway - so it's not as though Contraband is a super terribly BAD card here.
I'd like to draw particular note to fairgrounds here. Not only is it a nice $6 target, and a good source of alternate VP, which can lead you to have them only block one of your 6 VP targets in the endgame, leaving another free, but you also want most every card here. So no matter what they block, there's going to be something else for you to pick up. It also makes pretty good use of the +buy in this way.
Here is a game where I use contraband with a bit of an engine, mostly a big potpourri of stuff, to pump up fairgrounds and help me win a long game against ddubois. Of course, it also features Nobles, so it's really a model game for contraband being a nice card.
The other nice kind of card to help you is a trash for benefit card. Stopping you from getting gold is pretty good in the early game, but the real drawback is the endgame blocks. I mean, usually, you're able to pick up a couple components to help you if they block that gold - two silvers at least. Being able to get some benefit out of it later on, particularly with its 5 cost, is thus very nice. Salvager, apprentice, Expand (very nice because 7 cost cards give you options just like a lot of 6s do), I'm looking at you. Also upgrade effects (dark ages leads to increase in contraband usage? You heard it here first.)
Playing Against contraband:
The big thing here is to get a read on your opponent, and what it is that they're most wanting to buy. Now, at the very basic level, this is most often gold, then province later on. But this is very very often not the case. For instance, some other card can be more powerful than gold early on, you may need to cut them off from the KEY card they need in their action deck (the only source of draw or buy (well, other than contraband), or even more wonderfully sometimes, the only source of +actions). But most often, the story changes here in the endgame...
If you have a lead against a contraband player, you can do some very cruel things to them. If they need to buy provinces, first off, you can block them - this works pretty well from behind, too. But more importantly, there can really come times where provinces don't cut it, and they're going to need a lot of duchies. Block them from duchy. I cannot stress enough how important this can be. If they're going to need lots of silk roads to come back, block them from that. In a great twist of events, if they've played six goons, you probably want to block copper.
Here's a game against -Stef- where he uses a contraband together with his apothecary engine as... basically his entire economy (mine is not so much stronger). (Note: apothecary is very very fast, doesn't have tons of staying power). So, note what I do here. Early on, I prohibit gold, which seems pretty standard. However, I continue doing this for quite a while - indeed, there were multiple occasions where he drew his ENTIRE deck, played contraband, and I knew he was holding $10p, and I prohibit... gold. Now, why did I do this over blocking him from province? Well, I knew that eventually, if I needed to, I could block provinces later on. Because all the money in his deck was 10p, and 3 of that was tied up in contraband, I knew that he couldn't possibly by a province without one. Indeed, instead of province+apothecary, probably one of these times he should consider province+copper. Anyway, I have that in my back pocket, but more importantly, I'm expecting that he will start to stall out at some point, which in fact he does. If he never stalls, I'm probably just dead, because although I have a lead (which was really important too), I will almost certainly stall as well (and I do). Then we get to the really interesting part of the game, where I start blocking him on... duchies. Thing is, he's behind, and he needs to get duchies to come back, so this is actually fairly well a no-brainer. (I do make mistake in this game on turns 17 and 21 at least by not buying copper). But check out the key moment on turn 22. He plays 4 apothecaries and 3 caravans, of which I know he's drawn at least 3 coppers from the apothecaries, and some other specific stuff. He plays contraband, and I sit for a good while, thinking over what to prohibit. It was a very close call, but I didn't think he had the cash, and blocked duchy. Indeed, at least as important as that, I thought that if he DID have the cash for a province (which would have been game-winning), I was very likely to lose anyway, based on all the other goodies he'd be able to buy there. This is another important principle of what to prohibit: if there's two choices, leave the one open which, if they have the money for it, puts you in a worse position either way. I.e., if you need good luck to win, play for that luck.
Also, playing from behind, in a situation where they can three-pile end, you absolutely need to be able to use that contraband prohibition on cards from a pile for them to run out. I'm pretty sure I've blocked curses before (though it's very hard to look for that kind of log). In these situations, you're very often lost either way, but you need to at least give yourself a chance.
Keep your wits about you (this is a bigger problem on isotropic than IRL) - if they remake 2 cards, play contraband, well, they only have 1 card left in hand, and if it's early enough in the game, there's a good chance you KNOW they don't have a gold - so don't prohibit a 6-cost, as you already know they can't buy it. Similarly, if they reveal their hand, for menagerie, for a bureaucrat, for whatever, pay attention. If they put back a card, pay attention to what that is. Pay attention to where they are in their reshuffle, and what they've already played/discarded. Most important, pay attention to what's in their deck. You want to know what they have, so you know what they're capable of, and you want to know what they want to get, so you can stop them from getting it.
Overall, it's not a very good card, but a lot better than the plague some people tend to think it is, in the right circumstances. And the kind of game it can create is a lot of fun, and can make for quite a lot of skill, and a kind of skill you don't get out of many other dominion cards.
Works with:
Flexibility!
Apprentice
Upgrade
Salvager
Hoard/Harem/Nobles
Fairgrounds
Expand
Alternate VP
Goons (ish)
Engines where you aren't drawing your whole deck and you aren't stuck to needing one particular component
Doesn't Work with:
Hunting Party
Menagerie
Crossroads
Getting Stuck needing one thing (a la a lot of big money strategies)
Venture (which can force you to play it at an inopportune moment)