My session today that just ended is Exhibit A on why crit-fails are bad (and also a demonstration of poor strategic choices, and unnecessary PVP):
Our rogue crit-failed himself to death. Our sorcerer crit-failed himself into hitting our conquest paladin, who smited him back; the sorcerer then sulked and stayed back for 90% of the next fight. Our bard snuck off ahead and aggroed a billion zombies before getting herself cornered by them and dying.