Birkenstocks are the most comfortable sandals in the entire universe.
They're simply built-- just rubberish soles, leatherish straps, some little metal buckle things and cork, not space-age construction by any means-- and usually backless. They have, at first, a simple generic contour that more or less fits almost all feet.
But the key comes in the cork. Most of the sandal is cork. And even though the birks will fell weird and floppy and stiff for about a week, after the first week everything changes. Because after you've worn the birkenstocks for about a week, the cork starts literally breaking down just a bit, and literally reshapes itself to fit your foot perfectly. And thus you have the perfect sandal, designed exactly to your foot, fitting exactly to your foot, to an organic, perfect way that could not possibly be custom-designed by non-natural means to feel right to that degree.
If you get some Birks, make _absolutely_ sure that when the shiny stuff on the exposed sides of the cork wears off, you buy and apply some more. Failure to comply will result in the dreaded Birkenstock Rot. Treat your birks nicely, however, and they will last literally forever. I have heard stories of people using Birks that they have owned for upward of twenty years that are still in perfectly good condition.
I hate shoes. If only my high school didn't require shoes.. then i would be able to wear my birks all the time.. but as is i must be limit the bliss of my feet to weekends.