When I was 15 I had a girlfriend in high school that owned a pair of Birkenstocks sandals and I remember how comfortable they were, even over her own foot indentation. I would always ask to borrow them ;)

Recently when I went looking for a good comfortable house shoe that I could also wear out casually, a girlfriend of mine (that owns a pair of Birkenstock clogs), highly recommended the Birkenstock clogs/mules.  We wear the same size 7.5 and she suggested getting the 38 Narrow -- she was right, they fit like a dream and they're very comfortable. I suspect they'll only get more and more comfortable as I wear them. My only complaint is that in the meantime the leather upper rubs the tops of my feet, it's not by any means painful, but after a few hours my skin is pink and irritated. I think (hope!) that this gets better with time. There's also a squeak on my right shoe. That sucks.

These shoes run very true to size. If you have a slim foot, skinny toes, or if your toes don't fan out much, I would go with the Narrow sizes (otherwise your foot will swim inside, even if you choose the right size). Birkenstocks are most often worn without socks.

I got them in black suede (it's more versatile) so I could pair them with more of my clothes, and I chose the Bern style over the Boston because I think they look more feminine.