Ms. Minto's book is a classic example of a gem in the rough. Yes, it isn't visually impressive, and yes it is well worth using. Note that I said using, not reading. You won't realize the value of the methods Ms. Minto presents if you don't try them out; repeatedly.
My suggestion? Go to the chapters that contain examples first, and read them thoroughly. Then find a short report or memo you find particularly opaque, and read the book cover to cover, occasionally referring to your example. Finally, go back to those sections of The Pyramid Principle that apply to the example you found and start re-working it per Ms. Minto's methods.
When you're done, do it again, with another piece. Then re-read the book at least once a year. It's worth it.
Anyone can build a jumbled pile of stones. It takes time and practice to build an orderly structure that will stand on its own merits. The Pyramid Principle will help you become a verbal architect. I think it is well worth using!