well, the opponent is going to play around this the same way he plays around Mirror Entity; if possible, drop the most worthless minion out there. the positive side is that you get 2x of the card with the battlecry (so if opponent plays Novice Engineer and Swashburglar, that's a huge improvement to Mirror Entity result) the negative side is that you don't actually get the minion into play right away, so sometimes the opponent actually might be alright with giving you huge cost minions into hand knowing you have little or no time to play them out, since unlike Entity you didn't get nothing into play, and you gotta deal with the new card.
it does create an interesting scenario where opponent is split between playing the huge minion or the cheap ones. fun card. don't think it ends up in a deck (why would you steal your opponent's creature cards in Mage, when you have better things to do) but certainly will spark some moments.
Sure it looks good at first, but everyone with a bit of experience will try to play around Mirror Entity unless they're facing Freeze Mage, therefore you'll pretty end up with 2 small minions, but that's still 2 cards.