Summary
The Dice Man is a well-written piece of fiction, but, in my opinion, it has some problems. I will go through the problems one by one. There will be spoilers.
The book's main character is named Luke Rhinehart, who is the fictional "author" of the book (as exemplified by the pen name the book is published under.) In summary, the book is about a psychiatrist who feels unfulfilled by life. He is bored of his job, he is bored of his family, he doesn't have much interest in his work as a psychiatrist or the book he is writing on sadomasochism. One fateful evening, he notices that a six-sided die has been left out, and is obscured behind a card. He vows to himself that if the die is a one, he will go to the apartment downstairs and rape his best friend's wife (whose name is Arlene). If the die is anything else, he will do nothing. He continues to make decisions of this caliber with the dice, and ultimately his decisions get progressively stranger and more damning until eventually he ruins his life (he's completely satisfied with this, being no longer bored.)
If reading that summary made you say "what the fuck", I guarantee that you will have dozens more of these moments if you read the book. There's a plethora of sick sexual things in the book that are very obviously designed to be humorous, but only a sociopath would find comedy in them. Luke Rhinehart is obsessed with sex.
To the writer, women are sexually submissive
Arlene is one of the "problems" in this book, as are many of the characters. As aforementioned, Luke vows with his first dice decision that if it is a one, he will rape his best friend's wife Arlene. The die is a one. Lucky for him, Arlene is seemingly completely okay with this. She doesn't understand what Luke's intention is at first, but once he makes it obvious that he intends to rape her she submits completely willingly and even leads him to the bedroom. They later have an affair in spite of their respective spouses, and they regularly engage in consensual sex.
One could hand-wave this as simply being a grim quirk of Arlene, but the problem with Arlene being submissive to and enjoying being raped is, much more largely, a problem with the author's attitude toward women, and this is later exemplified in other women in the book; the writer views women as sexually submissive.
I can give a specific example. Somewhere toward the middle of the book, Rhinehart attends a work party hosted by a famous psychiatrist. He assigns character-personas to different faces of the die, and rolls a die every ten minutes to determine which "character" he will play at the party for those ten minutes. One out of six of these characters is, verbatim, "an uninhibited sex maniac". The other two options that manifested at the party were "a gentle Jesus" and "an absolutely honest dice man". At this party, he rolls the "sex maniac" persona. He corners a woman named Miss Welish in the hall and starts to sexually harass her, making comments about her breasts and her body. He then realizes that mere words do not align with "uninhibited", so he tries to hold her down and kiss her. He gets kicked in the balls. Fair enough. Later in this very same chapter, at this very same party scene, him and the aforementioned Arlene are going to disappear into the bathroom at the party and have sex. They offer Welish (who was earlier harassed and assaulted, much to her own displeasure) to join them. She drops everything and joins them, somehow suddenly completely willing to have a threesome with two people she barely knows. Why?
I can give another example. Rhinehart is seeing a young woman in his clinical practice named Linda, who is bored with life, views men as uninteresting, and has no future plans or goals. Rhinehart's dice tell him to make a sexual advance on Linda. He does so, and they have sex in his office. No questions asked, no protestation. Why?
Why? Because the writer views women as sexually submissive, and this manifests itself in the characters.
Everyone in Rhinehart's life abandons everything to the dice. It makes the characters feel one-dimensional.
The dice tell Rhinehart to leave his wife Lilian and his two children, and to not come back. He packs a bag and disappears from their life. Shortly after this, he starts doing "dice therapy" with people in order to initiate them into the "dicelife"; he wants other people to make all their decisions on the roll of a dice. The idea of him starting a small cult isn't unreasonable. He starts with Arlene.
He goes to the aforementioned work party months after leaving Lilian (his wife). She makes an appearance at the party, and confronts him, asking him why he would leave her and his children. His die-assigned role at this time is "an honest dice person", so he explains everything to Lilian. He explains that he started making all his decisions based on the dice. The dice told him to leave Lilian. It was outside of his control. Lilian is devastated. She physically attacks him. She cries. She is angry, and thinks the dice life is stupid.
Some months later, the dice tell him to go back to Lilian and re-seduce her. These months, he has been converting all his patients and everyone he can to "the dicelife". He has not had a single interaction with Lilian. There is absolutely no logical reason whatsoever in any way that she would think the "dicelife" is a good way to live, much less endorse it. But, lo and behold, when he visits Lil after all these months, she rolls a dice and then shoots a gun at him. The dice told her to shoot him, and she had absolutely zero protestations. Did she already want to kill him, after the grief she caused him? Maybe. The point is that, at this point and later in the story too, she is completely inducted into "the dicelife". Why?
For really no reason at all. It seems like an insult to her agency as a character.
His best friend and fellow psychiatrist Jake Ecstein learned that his wife had been cheating on him with Rhinehart. He confronts Rhinehart, and Rhinehart explains that it was because of the dice. Two chapters later, Jake is a "diceman" through-and-through. Why? Nobody knows.
Linda, his former patient who has sex with him, becomes a "diceperson". Why? Nobody knows.
There is way too much sex in this book
Much of the sex in this book is written to be mildly humorous and not written erotically, which I am grateful for, but if anything it just left me feeling disgusted and dirty after reading it. He writes rape, sex with clients (people he has power over), orgies, sex with hookers, there is just. So. Much. Sex.
Another nitpick I have with the book is that he writes a character who describes in great detail how he (the character, not Rhinehart) wants to have sex with and hurt small children. It's sickening to read, and I understand that it's just the character speaking and not emphatic of the will of the writer, but it made me feel very uncomfortable reading it and I ultimately think that he could have written the book separately. But that is a nitpick.
The book feels like it's missing plot.
BIG SPOILER here. If you want to read the book, skip this part.
Toward the middle and end of the book, there are a couple of interactions Rhinehart has with a group of radical terrorists. There are two such interactions. They are short, they are not very significant, and there is no development. At the end of the book, the terrorists (which Rhinehart has interacted with) come and kill everyone at Rhinehart's current venue. Rhinehart helps them attack and then helps them escape. There is no lead-up to this in which Rhinehart's involvement is made known, no build-up. It happens, and then the book ends.
Conclusion
A third of the way through the book, I was beginning to think the book was lagging. The idea of Rhinehart starting a cult of dice-people really interested me, which rekindled my interest in the book. Unfortunately, the book doesn't really detail him converting initiates or interacting with the terrorists that ultimately end the story. Everyone in his life that he knows or interacts with just suddenly drops their entire personality and lifestyle to become "a diceperson". The story left me feeling very unfulfilled. It's well-written, but in my opinion still lacking. It wasn't not-worth the read. That being said, I will not be reading the sequels or other books in the series (there are multiple). I just feel very neutral about the book; I didn't really enjoy it, but I wasn't bored out of my mind.