After giving Amazon's KDP Select Program a try for several months, I've decided to put my stuff back up at Smashwords. In a few days, everything should start showing up at various other online bookstores (B&N, iTunes, etc.).
The KDP Select Program allows for 5 days of free promotions within a 3-month period in addition to potentially paying out for Amazon Prime members "borrowing" enrolled titles. I've had a single instance of someone borrowing one of my titles, so the free promotions are really the only benefit I've seen.
I still love Amazon to death, but the return business following free promotions has dropped to almost zero in a very short period of time. Now, granted, simply being free on Amazon gets the books and short stories noticed, but I have a feeling that the majority of people downloading are people who download a lot of free content and whether or not they ever get around to reading what they've downloaded for free is questionable. This is not a complaint, just an exercise in logic. Hopefully there will be some benefit in this return to general exposure and expansion of choice.