The two straps adjust separately, so I finally got the fit right across the top of my foot instead of one strap being tight and the other loose. The Birkibuc upper has held up fine through a whole summer of yard work and quick errands.
Wore these straight out of the box for a full day at the beach and no break-in period at all. The Birkibuc upper feels sturdy but didn't rub anywhere, and my feet stayed comfortable even after hours on hot sand.