Hearthpwn

Big Priest used to be ridiculously slow, but the addition of the new cards in Darkmoon Faire such as Palm Reading and Insight make the deck SO much faster. Especially if you go second, you can play Palm Reading on 3, then coin out Shadow Essence by 5, and unless your opponent has a proper way to deal with it, you're almost set for the rest of the game.  

This deck is good against a fair amount of matchups in the meta. Here is what I would say:

Good Against:

Aggro - as long as you can keep wiping the smaller boards off with Spirit Lash [/card]and [card]Breath of the Infinite, you win by the time you play Shadow Essence. Zilliax is amazing at helping keep you alive against this matchup especially if you are low on health. This is true especially against Aggro Druid, Odd Rogue, and occasionally Pirate Warrior.

Highlander - Against both Highlander Priest and Warlock, they are tempted to play Dirty Rat especially since both archetypes are very dominant. But then that's mistake when they accidentally pull out yourBlood of G'huun or Scrapyard Colossus. Scrapyard is especially powerful against priests because even if they play Mass Hysteria, you will still most likely have a 7/7 left on the board.

Midrange - Almost exactly like the aggro matchup, make sure you are healing and clearing CONSTANTLY. Against Secret mage, don't spend too much time trading and clearing minions as you need to pressure face before they burn you down with Fireballs and Cloud Princes. The only exception is Arcane Flakmage, because it can wipe out your taunts if left unchecked. 

Counters - 

Odd Paladin - Yes, I mentioned that this deck does well against Aggro, but the exception is Odd Paladin. The deck has the capability of refilling their board with minions continously, rendering many of your removals useless. Never Surrender! and Oh My Yogg! can complete negate your removals and then they can just burst you down by turn 5 or 6. In any case, just be more cautious when using your removals.

OTK Anything - Any OTK deck has the potential to melt you. Especially the Aviana Kun Malygos combo. Here you want to play midrange style as the mana cheats will be very annoying. Against Quest Mage, same thing, but if you're facing the Antonidas Exodia, there's not much you can do. But in the case of the Giant OTK, Just build as many Scrapyard and Obsidian walls as possible to delay them.

Substitutions - 

Y'Shaarj, Rage Unbound - TBH I actually don't have him lol. But he is definitely always a viable option. Honestly, I feel like Blood of G'huun is just better because it summons a copy instead preventing permanent removal from the game.

Ragnaros the Firelord - Rag is one of my all time favorites, but his effect is a little risky and is a little to slow and doesn't have an immediate effect. Always consider him if you don't have better options though.

Idol of Y'Shaarj - You could put it in, but it's really slow, and honestly Shadow Essence does a better job at being efficient. The extra 5/5 stats doesn't really mean much. 

Catrina Muerte - To be honest, I don't know why I didn't put her in. The deck can function fine without her though. 

Conclusion: In the end, I recommend this deck if you want to try something different than traditional means. It goes fairly well against most meta decks.

(Also, to Big Priest haters: I understand you now lol )