I own 10 different styles of Birki's/Birkenstocks combined.  The leather Salina has been the toughest to break in and that's the only reason I didn't give them 5-stars.  The leather is durable and therefore, a bit stiff and doesn't give as much as the nubuck, suede or synthetic materials. They will probably age beautifully since they are the "luggage tan".

I finally resorted to a trick a shoe repair guy told me when I was buying stretchers years ago:  Spritz the inside with a 3:1 mix of water & alcohol (don't over saturate the leather) and put the shoes on while they dry.  I actually learned that you can do the same with the gel hand-sanitizer.  I never had to do this with other Birkenstocks but it worked like a charm on the Salinas.

If you're patient and work them through the beginning break-in you'll be very pleased with these.