I love this design for bouncing light into a room. The mirrors also makes the windows look twice the size. When I finally secure my London pad I'll probably look into constructing them, although on 3 sides only, with a window seat. The depth should make an ideal bookcase.
Also intrigued by the "fake" lights on the market. Haven't seen one in the flesh, but might save an otherwise dingy bathroom.
And along those lines, light boxes can give the impression of skylights. PLUS you're bound to have at least one friend who gets really confused about the skylight in the middle of a basement flat...