Cons:
MAY cause numb/hot feet, due to restricted circulation. However there seem to be lots of people using them with no problems.
A little harder to get into than 'click and forget' systems
not quite as efficient as clipless - slightly reduced ability to 'wipe' feet around the bottom of the stroke

Pros, that I can think of:
$20
easy to get feet out
Don't need fancy shoes/pedals combination, can use regular shoes

Overall, I think they're a good compromise for anyone who is afraid of being clipped in, or who is reluctant to get new shoes to go along with new pedals.