Spy is a card that does a variety of things. In a number of cases you are not putting yourself at any disadvantage by buying the card, as it's a cantrip that can at times remove an unwanted card from the top of the deck. In fact, as long as you do not use any more risky draw strategies that rely on terminal actions, Spies are only bad purchases in a couple of situations. These situations are only of a certain amount of concern, and potentially the benefit of playing Spy could be far greater.
What can Spy do for me?I'm glad you asked. Maybe the most relevant thing written on the card is the attack. With it, you can make your opponent skip past good cards like Platinum or Mountebank, potentially ruining their current shuffle if it's early. You can also guarantee that they'll draw a dead card like an Estate that was already there, though it should be noted that this isn't as cool of a thing since the same result would have happened had you not played the Spy. Still, it never hurts to be sure your opponent will have a 4 card hand, and if you can play multiple Spies to skip past good cards and also leave a bad one, your opponent will be crying at his or her computer chair.
Spy can also filter through your deck much better than other cards like Market or Woodcutter. You can remove bad cards that you don't want to draw, which gives you a cycling advantage of some value. Another great benefit of the Spy is that you can leave good cards for your next turn or even your next draw on the top of the deck. This is a level of useful when hoping to play key cards like Mountebank more often, but also some magnitude of powerful just for smoothing out general draws. Effectively, this ability is the reverse of Spy's attack played on yourself, but it can be of equivalent power.
Are there any drawbacks?A number of people would argue that handsize attacks hurt cantrips like Spy. By increasing the number of cards that do not give money, buys, or +Draw in hand, your options become more limited when hit by cards such as Militia and in other similar cases. However, one must bear in mind that Spy has the potential of filtering you more quickly toward important cards that are perhaps better than these cards you would have in place of Spy. As such, Spy can arguably overcome this drawback.
Another problem with Spy is that it can trigger bad reshuffles. However, since it's rare to have a deck with so few cards that you are always triggering these, more often than not Spy is benefiting the flow of your deck than it is harming it. As such, this risk is so small that in a number of cases it is negligible.
Perhaps the most common argument against getting a Spy is that it carries an opportunity cost. The $4 price point must always compete with cards such as Silver and Bureaucrat. It's also difficult to reach hands that have $4 when you are ridden with Curses and Ruins. (but bear in mind that a good Spy can help you get past those bad boys) However, when considering opportunity cost one must also remember that every card in Dominion carries some opportunity cost, so the opportunity cost in purchasing Spy may be of equal or lesser significance.
What have we learned?Spy is a card that should be bought in a variety of circumstances when playing Dominion, a game published by Rio Grande Games in America. It is useful for filtering past bad cards, leaving good cards on the deck, making your opponent skip good cards, and ensuring they have bad cards. Sometimes it might even trigger a bad reshuffle for them! You should always spend some amount of time considering purchasing Spies in any given Kingdom.
Works with:Any deck that can benefit from the general benefits that Spy provides