Set: Base
+1 Card
+1 Action
Each player (including you) reveals the top card of his deck and either discards it or puts it back, your choice.
Cost: 4
...It's Spy. I don't really have a lot to say about this one. It's never a truly awful play, but very rarely is it ever a good one. It sort of f***s with your opponent's deck cycling and sort of slightly improves yours, but not enough to the point that it's worth buying it. If you hit a bad card in their deck, or a good card in yours, then it isn't doing much and in the case of opponent's decks you can't continue to affect them with Spy that turn.
I'd probably only really bother with Spy if there are no other good 4 or less buys on the board, and I don't want Silver for some reason. Ironically, Spy is probably most effective in Scrying Pool decks of all things, which is completely nonsense because of how Scrying Pool completely outclasses Spy in every possible way except its cost. But Scrying Pool decks like actions, they love cantrips, and you can buy Spies when you don't draw the Potion in your hand.
The reason Scrying Pool is so much better, besides the obvious fact that it can draw your entire deck, is the fact that you check your own deck's card before you draw it. It's a subtle effect, but it matters, because it means you get to cycling your deck just a bit faster. Spy should do the same thing, despite the fact that it would make it even more like Scrying Pool, but alas, it does not. I still probably wouldn't buy it anyway.
For attacks that stop opponent's deck cycling, you can do better, like Ghost Ship. Same for cycling your own deck - I'd rather have Warehouse (despite it decreasing hand size), or even Farming Village. Spy tilts both decks slightly in your favor, but it's usually just not enough for me to care about buying it. Spy is mostly just a cantrip plus, and is best bought only in situations where you need cantrip plusses, like Scrying Pool, Conspirator, or even Minion decks.