Set: Prosperity
+3$
+1 Buy
When you play this, the player to your left names a card. You can't buy that card this turn.
Cost: 5
Usually a bad card, but don't sleep on it.
It's probably best to look at Contraband as a Gold that turns into a dead card in the Province/Colony race, but is better at buying cheap cards. Unless your opponent is clueless, or it's early in the game and the player misjudges whether you want Golds or Provinces, you can't use Contraband to buy Provinces.
However, early in the game, if you're careful, this card can be used as a cheap Gold to get engine pieces. That is probably the best use for this card - either building an engine, or building a junk deck. It's easy enough to deny provinces/colonies when you play this, but it's much harder, if not completely impossible, to deny the player kingdom cards.
Usually you'll want to buy Gold, but there are a couple of reasons you want Contraband. First, it's cheaper. 4 and 5 is the biggest difference in Dominion, but 5 and 6 is a pretty big difference too - if an early Gold can help you get some nice cheap cards faster, do it. The second reason you might want this is its +buy. If I'm not mistaken, this is the only source of +buy in the game that does not require an action to play, and that synergizes nicely with Contraband's primary use of buying cheap engine cards, allowing you to buy more stuff.
Because Contraband can't help you buy Provinces, and you need to buy Provinces (maybe with the exception of a Goons deck, in Contraband's case), this doesn't help a lot late game. I prefer Contraband greatly in setups where I can do something with it later, like trash it for benefit, or even use Mine to turn it into a proper Gold. If I can't I'm hesitant because having a Gold that turns into a Curse when trying to buy Provinces is kind of bad.
It also creates mindgames where you have to try to decide what to stop the other player from buying, which makes the game more fun I think. It's not a card I would buy very often, usually there are better 5's, but I've had some luck with this and it's a fun card to play.