My comment is in two parts: as a reader and psychology student, first, and as a fanfiction author, second.
It isn't a large book. It took me 8 hours to read it thoroughly.
It contains extreme violence, and I do mean extreme, worse than anything previously seen in Star Wars. Even by Sith standards, Darth Plagueis is sick. If "he could keep those he cared about from dying", that has to be the most twisted use of the words cared about ever.
As far as characterization and psychological realism are concerned, I've found nothing to criticize, and that's a pretty rare thing to happen when it comes to fiction. Especially fiction for which I had huge expectations.
Palpatine is a psychopath/sociopath right out of textbooks and true crime stories, fitting every point on the personality factor of the Hare checklist and most of the points on the lifestyle factor. The only error in the book is a confusion between mass murderer and serial killer, but it's a mistake even journalists make. A serial killer is someone who commits three or more murders with a substantial amount of time between them - weeks or months; a mass murderer kills four or more, all on the same day. Someone who murders his entire family in a fit of rage is a mass murderer, not a serial killer.
The dark side didn't make Palpatine "evil" - he already was. He always was. It was nature more than nurture, though from the hints we're given, nurture helped. But James Luceno writes "born evil" much more skillfully than JK Rowling did with Tom Riddle's backstory (which felt a bit like a lazy way out). It raises an interesting question: how far would Palpatine have gone if he had never met Plagueis? He may have been a regular criminal/murderer, more deadly than most because of his Force powers. He may have killed dozens, maybe hundreds of people before being hunted down by the Jedi - but he certainly wouldn't have been responsible for the death of millions. He may even not have killed anyone, because he only killed when he thought he could get away with it. He always hated his family, but he didn't kill them until he was convinced someone as powerful as Hego Damask would help him cover it up. So, here, nature plus opportunity was the recipe.
Contrarily to what Palpatine thinks in the novel, Plagueis's fatal error wasn't his failure to understand the human species and its emotions. Not quite. It was his failure to understand his apprentice's murderous hatred of all those who ever tried to control him. An emotion most humans don't feel, not to that extent.
Some reviewers have complained that the novel reveals too much, shattering the mystery that always shrouded the Sith in Star Wars. I disagree. Luceno has a talent for giving a lot of information while dodging the questions readers expect him to answer. For every question the book answers, it raises another. Luceno resorts to some narrative acrobatics to avoid answering questions fans have asked time and again: did Palpatine ever learn Plagueis's technique to keep people from dying? What is Palpatine's first name? How did he reveal his Sith identity to Dooku? When did he meet Padmé and how did he help her become queen? Did the Sith create Anakin Skywalker? We still don't know. The most Luceno gives are hints, for example for the question about
Anakin's origins. No direct answers.
It's a very good novel, but not really revolutionary for anyone who read between the lines in the prequels. I don't mean it's disappointing - far from it. My expectations were very high and the book has met them. But that's just it: it's exactly what I expected. Very few surprises.
The only truly surprising revelation in the book is that Plagueis was still alive during TPM. I don't think anyone imagined or expected this possibility. But when you think about it, it makes sense that Palpatine kept his "master" around as long as he was still useful to him, and got rid of him only when he no longer needed him or his help for anything.
Okay, now donning the "writer" hat. If there is one really annoying thing about being a fanfiction writer, it's having to rewrite a fic in progress for compliance with an updated canon. Of course, some people decide not to rewrite, but AUs were never really my thing. I got into the Harry Potter fandom after OotP came out. The two following books significantly redefined the universe and characterization and added a lot of new characters. This ultimately made me stop writing HP fanfic altogether.
I was worried the same thing would happen in regards to Star Wars, and when the Plagueis novel was initially canceled, I was relieved. The area I tend to focus on in my fics is mostly uncharted territory in the movies and the EU, so I could write a lot of things without risking contradicting established canon. Darth Plagueis has changed this, but much less than expected, and far from causing a permanent writer's block like the 6th Harry Potter book did in my past life as a HP fanfic author, Darth Plagueis has filled my head with fresh plot bunnies and a renewed motivation to continue and finish the stories I've been working on.
Yes, there are things I have to rewrite, but nothing major, and this time I don't mind rewriting! There were several scenes I kept rewriting and really wasn't sure about and now I know exactly how to fix them. And where before I would have had to explain my interpretations of characters and backstory, trying to convince readers of their plausibility, now I can just quote the only Star Wars tie-in novel that has been officially labeled canon. Or would that be a lazy way out?