One thing that has always bothered me about the Book Book is the potential for air flow disruption. The path of least resistance for cooling airflow, both in and out of the computer, is the slot opening that is created at the rear of the MacBook Air, when the lid is opened. Yes, I do realize that there exists a smaller opening for this airflow on the top of the hinge, right in front of the screen, but the Book Book will effectively hinder the superior opening, and also permit more "rebreathing" of hot air.
This concern may be lessened by a cooler ambient temp, or light usage of the device. However, if the system is used in less than optimal conditions, I can't see how the net result can be anything but higher operating temps. As we've noticed that quite a few users "peg the redline" on temps, use of the Book Book may be detrimental if you use the unit with it wraped up on the outside.
Other than that, they are awesome looking.