I have luxating patellas - in non-jargon, my knee  caps move sideways and up & down, not just up & down. Orthopedist recommended footwear with firm arch support, so when I put my weight on my foot, my arch doesn't collapse a little and put more lateral forces on my poor kneecaps. I've worn Birkis and Naots ever since. the Birkis at first were just butt ugly but have improved/evolved fashionably although I realize the Jimmy Choo and those red soled Laboutin lovers will recoil in horror.

-Birkiflor, synthetic suede is actually not bad, have always bought leather in past
-a little padding is even more comfort. I live in Texas and wear sandals most of the time. I learned that I wear the Birkis so much the soles turn into rocks. Supportive rocks, yes, but too hard. They last maybe 6 months of daily wear and the Birki flor make it much less expensive to replace.
-mind the width when you buy - each shoe has a footprint symbol on it - a filled in print means Narrow in Birki-talk but medium IMO for women, narrow for men (unisex). An outline means medium in Birki-talk but I found it means wide in women and I guess medium in men's. Imperative to make sure you get the width you like.
-always true to size. You can order in confidence that the size you bought before and like will be the same size, every time. Trust me, I've owned a mountain of these shoes (including a pair of pink rhinestone ones I had resoled before they turned to rock). You can try a pair in a store to find the best fit.

Update: I was Birki shopping and learned something new about this sandal. It has a new, soft footbed. No wonder it feels more comfortable. I had to try on a pair I had bought in Berlin with an original footbed to compare (because I could) and the soft footbed is absolutely better for my feet. I also have rheumatoid arthritis and have had plantar fasciitis and bone spurs in both heels, plus I made the tremendous error of having simultaneous fascia release surgery on both heels. Now I have lingering nerve damage, the kind that hurts, not the kind that makes me numb. I definitely prefer the soft footbed over a well worn original footbed, but a new original is very comfortable as well. I noticed a number of people do not like the soft footbed and complained that the product listing did not mention "soft." I have worn these sandals every day since about April and they are still noticeably cushioned. These footbeds seem to be very similar to what Naot uses in its sandals. RA = daily pain so I take relief anyway I can find it. I will buy only soft Birkis in the future.