I've tried both and it's flats forever for me...

Pros:

--Inexpensive. My BMX pedals were $20 and I love them.
--Total comfort on the bike and no you don't need still soled shoes
--Greater comfort off the bike with normal shoes.
--No need for a second pair of shoes
-- I feel safer with flats and had an incident the convinced me of this.
--Freedom to make micro adjustments to avoid injury.
--It just feels better to me.
--The satisfaction of NOT having bought something that is bicycle specific that you really don't need.

Having said all this if clipless does something for you that's great. Many people love them and if it makes them happier on their bikes that's all that matters.