Interesting. Will be awesome against Justicar Trueheart. I'm assuming it's not permanent as it's "copy" and not "replace", and the fact that it costs 6 mana make me to believe that this card is most beneficial at turn 10 where you initially use your own hero power, play Sideshow Spelleater, then use your opponent's hero power as it most likely reactivates.
The way I read it, this comes into play and instantly does the effect of your opponent's hero power. So, if you play against priest, then the battlecry is "restore 2 health," for example. So 1: your hero power is completely unaffected, and 2: you do not need to spend more for your opponent's hero power, this guy does it for free as his battlecry.
Am I reading it wrong? If they meant replace your hero power with your opponents, then I think they would have said that.
Only against paladin or warlock. Other than those two, I can't really think of a hero power that I would prefer over Totemic Call. And, if the meta is slow enough to play this, why not play Charged Hammer?