In the interest of full disclosure first let me say that I am a KDP user and have several Kindle books available.
I can honestly say that I have had some success. Now I will not be quitting my day job any time soon but I make a nice amount to supplement my income from my real job.LOL.
Now don't get it twisted I would love to sell a million books and get movie deals and become rich. But if it doesn't happen that's okay. Why? Because I got to publish my work and someone actually paid to read it. Someone else read my story! Isn't that what writers want first and foremost?
On Amazon KDP forums you will see many post attacking Amazon KDP. Now KDP does have some issues with reports and contacting support. I hate that there is no phone number and you have to go through their site ( email ) to get a question answered. Then there is the legitimate debate about KDP select which is a complete issue unto itself.
But to the complainers I would ask, " Is there any other easier way you could have gotten your book out and actually sold it?"
For all of KDP's issues I believe it's still the easiest and quickest way to publish a book. It has offered me the opportunity to fulfill a childhood dream and become a published author. Not just here in the United States but in other countries as well.
I sell four times as many books through KDP than I do with Smashwords, Pubit, and Scribd combined. I have not tried Kobo yet as I have just found out they now have a publishing method similar to Amazon KDP and Pubit.
Like many KDP authors I don't have a publisher or a huge marketing budget but I can do what I can through Twitter, Facebook, GooglePlus, and other online methods to market my books. I can help build word of mouth.
KDP gives me and many other authors more opportunity than any other eBook format or store before it to both publish and sell our books.
Is KDP perfect? No. Does it give many authors a chance to be published and earn money? Yes.
Isn't that what all writers want? An opportunity.
George Cook author of The Dead War Series.
P.S. I know some will strongly disagree with my post but this is my opinion based on my experience. All other opinions are welcome.