The Priestess is, for now, my go-to for competitive play. I don't run the Temple of Asyra in my spellbook. I have Clerics, but I drop them as afterthoughts maybe 1 in 5 games, so they are probably coming out, altogether. In my opinion, the Priestess is bogged down by these cards, and is much more brutally effective in the short game than people, in general, perceive. I was one of the people originally stating she's slow, but now she's fast. It takes time and practice to see what works, think outside the box, a bit, and bring out her true potential, but as she stands, now, I'm playing games with no Asyra in less than 45 minutes and netting close to zero damage on the priestess, herself. I do see her as a real tournament contender. She's stronger than the Warlock, by far, from my perspective, and while everyone else crows about him I just have to snigger.
Is she overpowered? No. There's nothing in this game that is overpowered. The balance in this game is exceptional. Is she very, very strong? Yes. In my opinion she is the strongest Mage, but, as with all games, people only need to tweak their builds a bit in the right direction to counter her, and then she must tweak her build to counter the counter, and thus the story begins.
-nihil