Ask why a 5th Dan Aikidoka who studied under Yamaguchi Sensei would drop everything to train with Sagawa. If Yamaguchi is one of the greats what did Sagawa have that he didn't have.
Depth pf material?
I had a well known 6th dan who also trained under Yamguchi tell me he has learned more here in two years then in the last 20 years in aikido and that I could quote him.
Solo training to change the body, internal power/ breath power/ inculcated martial movement with weapons and empty hand is a pretty potent mix to generate aiki.
Trying to convince someone it is different from aikido on the internet or through a book is a fools errand when five minutes in person simply ends all debate.
Everyone will have thier own views, for me the overiding power of the book was
1. To finally have someone from DR admit that Aiki power is from changing the body through solo training and not waza. (this after I was told I was full of it for years by those in and out of DR) Some still deny it to this day all while claiming the deeper understanding -for themselves.
2. To finally have a well respected teacher admit openly that he was holding back from his students till near the end of his life (even intentionally misleading them) and that it was Sokaku himself who told him to. Tokimune admited it as well. And all of which was also denied by many in the art who claimed (of course) that it was misunderstood. I would say a general sampling of the movement of the vast majority of DR people speaks for itself and removes all doubt about only a very few getting the real "it" that Sagawa was on about.
3. That Takeda told him to NEVER teach gaijin, as they were already big and it would give them an advantage. Something which has been heard in ICMA aswell.
So other than pointing to a truth about solo training to change the body to generate aiki, and that the aiki body was the goal all along and not the waza- to the greater body of martial artists still in hot pursuit *of waza*...the rest of the book you can have.