Decent in a control deck. If you can copy and kill your opponent's heavy legendary, you will be ahead in hand value. The problem is that you can just put a heavy legendary in your deck to have the same effect. You lose some versatility but is more consistent. Unless you want to copy a class legendary like Tirion Fordring. Meh. It's decent. Maybe some Control Priest will run it.
So, basically you pay an extra 2 mana to play any minion that they have on the board? Extra choice on "what minion do I need" could be good. Say choosing a Sludge Belcher or a Healbot... or choosing Dr. Boom or Thaurissan.
I'm surprised at the tepid enthusiasm for this one. This seems like the type of card that all the Priest Timmys go crazy for. It's a VERY good tool for control matchups, allowing you to run an extra Sylvanas or something. There's not really many cards in aggro that you would want to spend 2 mana drawing a copy of.
SWEEEEET! I had a deck entirely themed around stealing and copying opponents minions and spells and I called it convert priest and now they straight up release another spell that would fit in that deck and it itself is actually called convert! WOLOLO!
Deathwing problems? No problem, just take 12 damage to the face than play Deathwing a turn later... This really does work so well with lots of legendaries and is a good answer to other class drops. If you manage to delay your opponent's Dr. Boom to turn 8, you can turn 9 Convert --> Dr. Boom! Such value.