I use keen clipless sandals. I like being securely attached to the pedals so my feet dont slip off when the pedals are wet; 20 years later I still have scars on my shins from that. I have no intention on ever going back.

However, I will rock flipflops with egg beaters when the wife wants to go on a bike ride though. However, that's usually twice a year on a sunny day with smooth flat ground, at 6.3 mph, for a total of about 4 miles.