I actually used the 8th edition. This textbook was okay. It excelled in its examples of how research could be used, though some of them went on too long, as they were most helpful in detailing the nuances of confusing terms.
But it was also dense and hard to read. The organization of it is relatively confusing and diagrams were not great, nor did they add to the readability. Pithy definitions of terms were not truly included, but generally assumed or hard to find in the pages upon pages of examples. I had to cross-reference terms constantly to be sure I understood the underlying definition of what was going on, though I did get a good understanding of how and why you would use one design over another and so on.
If you need this for class, I definitely recommend renting rather than purchasing. I am not keeping it for my personal reference library when better clarity can be found elsewhere.