I don't like the program because I don't think it's a good idea to give that much control to a single retailer. I also think authors should have their ebooks available in as many places as they can manage. This gives potential customers the option of buying your ebook in their preferred store.
But I will admit that I don't know everything about ebooks, so I've been reading what authors say about KDP Select. In at least some cases, it might be in the author's best interest to join the program.
Jeff Bennington offers the best argument in favor of joining. If you go check out his blog, you'll find that he did the math. He signed up with KDP Select because the Kindle Store accounted for 97% of his ebook sales. Out of nearly 6 thousand copies sold (since he started in March), over 5800 were sold in the Kindle Store. Giving up on the other markets isn't going to cost him anything.
I suspect that's going to be true for a lot of authors who join, which makes perfect sense. The whole point of self-publishing is to retain control so the author can get the greatest benefit. Self-pub is a business, so this should be treated as a business decision (ignore all the naysayers). If the best deal can be had by giving Amazon an exclusive, then do it.
But first authors need to ask themselves a question: What are the chances that KDP Select will pay me more than I earn elsewhere?
On the other hand, I don't expect the earnings from KDP Select to amount to too much for any single author. For the sake of simplifying the math, let's say that the total number of books hits 50 thousand, and that they are all checked out equally. That probably won't happen, but if it does then the mean earnings per title would be $10 a month.
If you already get more than $10 from outside of Amazon then KDP Select is probably not the best choice.
To be honest, I'm not certain whether KDP Select is the best option in Jeff's case. He's selling his ebooks for between $1 and $3, and while I don't know how much he's earned elsewhere, it seems likely that he might be earning less than the nominal $10 a month I mentioned above. But it does appear that Jeff is an excellent edge case for testing the value of KDP Select.
In any case, most everyone who condemned KDP Select yesterday (including me) really should have waited for authors to weigh in. We' also should have waited 3 or 4 months to see what income it can generate for the average author. That's the true deciding factor for whether anyone should sign up.
I'd like to hear more about what authors think of this program. Little that is said by non-authors matter in this case; it's your situation that is important. Are you signing up? Why? I'd like to know.