My analysis will, of course, be short and without a bunch of graphical data. I choose to just post my own personal experience.
The two books I selected for the program were: Window Treatment and All the Right Places.
The number one advantage to enrolling in the program was it allowed me to list my books free for a total of five days out of the ninety day enrollment period. While I saw a marked increase in purchases while the books were listed as free, they also fell under the free ebooks category for sales standings. When they reverted back to their ninety-nine cent price, they saw only a temporary rise in position.
As most of you know, position is everything in marketing ebooks on Amazon, or any other online sales channel. Once a book climbs to the lofty top one hundred position, it tends to stay there awhile. I've yet to see any of my books (except The Fifth Beauty, offered free) make it into this top one hundred position. My sales tend to be slow and steady.
The major downside to enrollment in the KDP Select program is the exclusivity clause. Books enrolled in the program cannot be marketed through any other sales channel. For me, this one disadvantage far outweighed any positive the program may have provided.
I believe the potential sales I lost at Barnes and Noble, ARe, Apple iBooks, Kobo, Diesel, Smashwords, and other markets were more than any possible gains through Amazon, Although their literature made it seem as though my sales would skyrocket because of the the ability for their Prime members to "borrow" books in the program.