Possession is easily one of the most controversial and game-changing cards in Dominion, and I've often heard it described with disgust by many players. Most Dominion cards require you to take advantage of the power of your own deck, and thus are relatively easy to come up with a strategy for. But Possession usually requires you to take advantage of your opponent's deck instead! How can I tell if Possession is a good buy and how should I use it?
(Links to the Dominion strategy blog are fine, but please make sure your answer actually contains a good complete answer and not just the link. :) )