This is gonna be a really hard to card to play. I already see myself constantly debating what turn to use it on. I also like how it fills gaps in your mulligan. If you end up with a ton of high cost cards, you can play this when there's nothing else so you don't fall too far behind on the board.
I think having a minion always on curve is amazing.
Epic cards are always heavy RNG based. If you use this on turn 5 and get a 3/3 healbot it's absolute trash or use this on turn 10 and get a 6/6 C'thun. You need some luck for it to be amazing and snipe a OP card. This will be better in Arena then constructed where it won't see any play it doesn't make sense to run this when you can put the real thing in your deck. Arena is a different story because of the legendary swing factor.
But in large this card is below average to average in most cases.
As an earier poster said, one of the biggest drawbacks of Priest is that their hand often ends up being super reactive and they can't take the initiative on board. This card will make sure that you can always get a big minion when you need one.