How is the Quality of KDP Books?

Books published through KDP can be just as high-quality as books published by traditional publishers.
The print quality is what you would expect from a traditional publisher. Amazon uses industry-standard materials and printing methods. There are two choices for paper quality: 55#, which is the less expensive paper and is essentially the same as copy paper, and 60#, which is better for color photos and illustrations as it is thicker and better finished.
Cover design and formatting quality are more in the hands of the author. The material used in the KDP paperbacks is the same as you would find with a traditional publisher. The same holds for their hardcover versions.

The Cost Factor

KDP allows for lower pricing than traditional publishers, but this doesn't necessarily reflect quality. Authors are able to maintain higher margins while still offering competitive prices, a primary reason many established authors switch to KDP.
The best way to know before broadly publishing is to order a proof of your book. This will allow you to see the paper quality with the book in hand and be able to make a decision based on your own preferences.