If this is real, I think this card will pretty much see very little play in constructed. You wouldn't use this on a small minion and if you use it on a big minion they can just use the big minion on one of your big minions or at least a small one to weaken it severely. If you don't have anything for them to run the minion into you're gonna take a big smack to the face. Also it's very susceptible to silence. I'm not a huge fan, but maybe in arena it'll see some play.
If you have minions on board, this spell just forces enemy to trade. (Corruption also does the same for 1 mana) Can easily be countered silence. If it succeeds, it's still not good enough because you need wait one more turn due to minion's exhaust status. Instead of "at the start of your turn," they could use "at the end of your opponent's turn,". That would make this card playable in constructed. Now, it's just a spell with unique effect.
So your opponent has 1 more turn to do anything about the minion you pick, and after that you gain control and still can't attack with it for a turn? Jesus that's super slow, but there are situations in which this can be good.
Too slow. It'll give your opponent the chance to kill or even silence the minion before your turn starts. Mind Control is just outright better.
And... the card you just listed also costs 4 more mana.
This card is good since priest can quite reliably make this card cost 4-5 mana without being really in fear of losing their Radiant Elemental. In an expansion where Deathrattles are important, this card is good since you don't exactly want to silence your own stuff and the idea of 'they can just trade it away' ... well, then you probably used the card very badly if you allow 'em to do that.
Is it 'brilliant'? Nah', but its damn good because its a Mindcontrol for 4 mana less in addition to Radiant Elemental reducing it further!
I'm surprised that so many people think it is decent. I think it is interesting, but its power level is ABYSMAL. Priest has already got a lot of situational cards, and this is extremely hard to pull off/too unreliable.