I have owned two pairs of these for about a year - one in black and one in brown. A few points:

1. The brown definitely has a more casual look than the black. The wear on the brown shoes is more appropriate for a more informal outfit. That being said, I use the brown frequently for business casual situations.
2. If you shine up the black with some polish, you can really add some shine for a more formal look. They are not very glossy out of the box.
3. These really are incredibly comfortable and light. The slip-on style makes airport travel a breeze as well.