The only correct way to play yogg is in nukedecks. Just have him as a last resort when you are about to loose anyway. I like it best in mage where you can delay for 2+ rounds with iceblocks.
He sure ended my hope of winning when he
a) Gave my opponent some life
b) Left his board alone
c) Killed himself
d) Starfired my face
Although in his defense he wasn't actually my Yogg, I got him in my hand thanks to a Shifting Shade so he was probably just helping out his owner all along.:p