I do my biking in the summer in sandals and in winter in boots. I have size 16 feet, so bike shoes are not an easy or affordable option and I have never worn them. My longest day was last summer in sandals, 117 miles towing a canoe. I did have a tailwind, though I also gained about 1500 feet over the day. I find sandals very comfortable for riding, they let the air and water in and out easily and they make it easy to hop into a convenient river or lake to cool off without having to worry about socks and shoes.
Zach