Ahh, Withdraw. One of the many underrated cards in the much-maligned Prophecy expansion. Along with the excellent Curfew, this is one of my favorite bounce spells in the game.

All the comments above have really gotten the key points: you needn't wait for your opponent to tap out to get effective use of the card, though bouncing two of your opponent's creatures for 2 is excellent. Bounce their creature then bounce your own (by electing not to pay the 1) either in response to a hostile spell or recharge an ETB creature. Lots of fun ways to use the card.

@tavaritz: definitely a good point, there. Withdraw shouldn't be equated with Unsummon.