Compared to Mirror Entity this card leaves you with a 1/1 behind on tempo. It also doesn't benefit from the large amount of removal mage has at it's disposal (in the sense that you can remove the minion played onto Mirror Entity and further benefit from your copy of it).
On the other hand it is largely better against combo pieces, like Thaurissan, Malygos, Chargers and other non-Battlecry minions that impact the board the momment they are played.
If the meta is extremelly combo-oriented this could see play over Mirror Entity, otherwise I don't think it's useful.
Too many people think this is good. It's playable in decks that haven't enough cards yet, but beside of this, this is actually one of the worst secrets. Against aggro this mean's nothing, and against Hunter this might be actually negative value since it gives him Beast-Synergy. So in the end this will be countered like mirror entity; only that with Mirror you actually gain a minion; and that's always better.
I must be missing something, as this is one of the first cards I bothered to rate that I actually rated "bad".
Isn't this just an expensive Snipe? All it does is transform your opponent's worst minion into a 1/1 sheep. It's so easy to check for. I suspect that most of the time, this turns a 2/1 into a 1/1. Of course a lot better in a pure control meta, but that won't happen.
screws with late-game decks, which is something to have in account. and this will see play in wild to prevent people from playing valuable deathrattles..also.. PRAISE YOGG!
Against "late game decks" you actually want Polymorph, and both of them are better than this. With this secret you can't control what you poly, it does give negative value and so on. And against Deathrattles: if the mage has a secret, nobody plays a valueable deathrattle already: you don't even need to change your playstyle compared to mirror image, since they are both the same: only that you need to deal with mirror image with the minion afterwards, and that can be very awkward. In Wild you don't even need to deal with the enemy minion, you still have a 1/1.