Ever walk up to a door and pull on the handle, only to discover the door is push-only? Do you really know how to operate your clock-radio? Doors may be pretty, clocks may look cool, but when we have to actually use them, the measure of their design changes.
Same with websites. A website may be pretty at first blush, but can you find what you're looking for? Can you do what you want to do? Can you even figure out what you're supposed to do?