Ive been using SPD type pedals of various types for 15 years racing mountain bikes and cyclocross and commuting on fixed gear bikes for about 9-10 years and haven't had problems with them. Just make sure to keep them well adjusted and CLEAN (dirt or grit will wreak havoc on their ability to stay clicked in)
As for Look pedals on a fixie IMHO it's too much problem trying to flick the pedal around while the cranks are turning all the while trying to avoid obstacles like cars and stuff.