I bought 2 pair two years ago in different shades of brown because I found them so extraordinarily comfortable and did not want to take the chance of wearing one out and not being able to find an exact replacement.
With this same thought in mind I recently acquired a third pair. While almost as comfortable, the straps are slightly less comfortable the and shoe bed feels a bit smaller. I also acquired blisters on the center of the balls of both feet after only walking about a mile or so. This did not happen with my other 2 pair -- I can only surmise that this resulted from the slight change in sizing.