Just went to the Mayhem Festival yesterday.
Best by far: Suicide Silence, 36 Crazyfists, Mastodon, The Red Chord (they did a fucking wall of death!), Machine Head, Slipknot
Worst: Dragonforce, Airbourne, Black Tide, Dead Broke
Mediocre: Walls of Jericho, Underoath, Disturbed, Five Finger Death Punch