I decided to splurge on a pair of Birkenstocks this summer.  I had one pair of Birkis that had just a single strap across the top, in a white and blue patternbut I wanted a real leather pair this time.  So I got this one at a slightly discounted price compared to the actual Birkenstock website.  I was worried about whether they would be genuine, and so I checked the rating of the company selling it (notice that all of the colors and sizes may come from different sellers) and after I ordered, I checked online about how to inspect the shoes for knock offs.  I received mine today (they shipped very quickly from a seller in the UK) and I inspected them carefully, including the box, and they are indeed genuine.  The leather straps are nice and thick, with the proper numbering on the underside.  The soles are correct. The buckles are right.  The box was correct (although crushed, oh well).  There was a little bit of whitish glue at the toe of one but I scraped most of it off.
The sandals themselves are comfy!  I wear a size 9.5-10 in regular shoes, and a size 9 in many sandals, and I wear a size 40 in Birkenstocks.  I also run borderline wide in most shoes and like wide toe boxes.  The 40 fits perfectly.  I do have a kind of fat big toe, so I am going to have to kind of stretch out the toe-hole as I wear them because that feels a bit snug.  But I got leather in hopes that it would eventually do that.  I actually like these a lot better, right out of the box, than my Birkis because the straps hold them on my feet better than the single strap I have on my other pair.
Anyway, very happy with my summertime splurge and hoping they will last me a good long time!