I have just passed the 640-863, and decided that would be a nice opportunity to share my first thoughts.
The exam is quite nice, covering a very broad area of knowledge. If you stick to the Cisco recommended reading, you should be fine (still requires major time investment to substantially benefit from this knowledge)
The exam was not easy at all I should say, at least, it did not seem to me. I have still scored quite well.
Overall, I feel that the Design track is a great addition to the rest of Cisco tracks. It is very different from CCNP or CCSP etc., as it focuses on the procedures, best practices rather then the implementation. Now looking back at the exam I realized it is not worth taking it unless you have already good implementation knowledge and hands on approach (something I already have). It sums up years of experience. It is somehow focused on the Cisco equipment, however, all the major concepts are generally applicable and are very vendor independent.