In mid-March Dark Doses finishes its first 90 days being offered exclusively via Amazon's KDP Select program. The results have been less than spectacular. Thus I won't be renewing it and will release the book to other venues like Smashwords.
I think KDP Select can be useful, particularly for authors who have a portfolio of books and they want to set aside 1 or 2 as magnets to draw readers to the others. Here's an example. Say you have a 5-novel series for sale. You could enroll just the first one in KDP Select as a way to hook and draw readers to the rest of them. Or you could write 2 or 3 short stories or novelettes as tie-ins and put those shorter works into KDP Select while keeping the novels out.
Beyond that, I'm not seeing much value for the everyday author.
The 'borrow' feature for Amazon Prime customers, while interesting sounding, seems to me to work best for books more in high demand. Not many folks would want to burn their limited borrowing opportunities on lower profile or unknown books. So the bonus bucks Amazon credits to KDP Select authors for borrows is fairly constrained, though Amazon likes to make noise about the monthly budgets allocated to borrows.
Finally a thought on the 'make available for free' feature. There are beaucoup processes and web tools now that enable Kindle owners to soak up the tidal waves of KDP Select freebies. It's a slick arrangement for Kindle owners. Without risk or outlay, you can grab 100 stories in a day, scan the first 3 pages of each, and discard all but the top 3 that you feel are worth reading start-to-finish. By the end of a week, you've crunched 700 books down to 21 that are locked into your To-Be-Read list... all without spending a dime. You can literally keep that up until your Kindle cries: "Uncle!"
What does the KDP Select author have to look forward to in this? A shot at discovery. Perhaps a Like or a meaningful review. If you're fortunate, your book just might catch-on via chatter in social media circles or from a high-traffic blogger's comments. But I would not recommend holding your breath for any of that.
I don't regret the experiment. It was worth a shot. When the 800-pound gorilla gives you a way to rearrange the bananas, you have to see if you just scored a banana bonanza out of the deal. In the case of KDP Select... not so much.