DD did CVW January 2015 and NASS 2015. CVW is indeed free except for transportation costs. Potential candidate is called a "drag" and 4/C is assigned as "dragger". In DD's case, dragger and her roommate were both D1 athletes and did not have a normal schedule (one up super early for practice; the other up super late for practice). They were both nice enough, but DD did not get good flavor for what life would be for a regular plebe. She was able to attend a couple of classes, but didn't take part or witness any formations or fitness routines. This experience probably greatly depends on "dragger" assignment. It was crazy cold and, overall, the dreary days of winter seemed to make the mids less-than-enthused about life at the academy.
For DD, NASS was much better experience because it gave a better feel for academy life. While there's not much academics at NASS, you do attend seminar-style classes in your areas of interest, and you get a bit of an idea of what it's like to be part of a company.