I bought my first pair a few years ago and was introduced to them by a colleague who swore by them. I didn't really get what all the fuss was about and always considered them to be a bit overpriced for what is essentially a summer shoe. How wrong I was!
After buying my first pair and having worn them to death, I promised myself I would get a smarter pair. I've just had delivery of the Gizeh in Black Patent and they are everything I wanted. They're smart and can be dressed up or down and are SO comfy. I'm currently 31 weeks pregnant and have had to let the strap out to accommodate my swollen feet and they are STILL comfy!
I would imagine for those people who don't like 'thong' or 'toe post' designs, these wouldn't be their thing, but I absolutely love them.
Like most shoes you do have to be careful of sizing, so I would suggest trying some on in a shop before buying online. I'm usually a UK size 8, and I wear a Birkenstock EU size 40 as they are quite wide.
Don't bother with cheap immitations- Birkenstocks are wonderful and will last for years if you look after them.