Hearthpwn

If you're playing him while behind he'll either turn the game around immediately or just end an already lost game. As others have already stated he's very likely to clear the board, draw cards, and drop secrets when you play him. Used him to come back multiple times after an ice block as mage with a control deck. If you're playing a spell-heavy deck that can make it to the late game and have the slot to spare then its probably worth it just for being able to completely turn a game around (Plus the inevitable maddening laughs you'll develop from some of the crazy spell combinations.)

Quote from aiicaramba >>

Quote from reggin_bolas >>

I've noticed the following trends:

He tends to stack secrets on your hero, rarely if ever on the opponent. 

He tends to kill off the board almost always

He tends to force draws on both players.

--------------------

I wouldn't play him close to fatigue. 

 Yogg-saron plays all spells as if it were your spells, so secrets will always be yours. He will, however, chose targets randomly. Thats why targetable spells can be on any character. Things like flamestrik will always be on the enemy side of the board. Beneath the battleground will always put them in your opponents deck and call of the wild will always summon the minions for you.

I had him DOOM! a full board, draw me in fatigue, then roaring torch and gang up on a huffer to fix my fatigue problem :P

I´m reall unsure about his competitiveness, but it certainly is one of the most fun cards ever created. Every time i see him i just laugh my ass off.

It never works, but heres one time it did

https://www.youtube.com/watch?v=WK6Q_aAYxu4

10% Screws me over

60% Turns odds in my favor

30% Game win