I'm a bit conflicted about this book because while I liked the story, I didn't enjoy reading it. Although having finished it I am extremely tempted to re-read it as I suspect I might really enjoy it the second time around. One of the things you really need to do, in order to truly enjoy a book, is to trust the author. This story is about some normal folks becoming really super-intelligent, and as Jo says, this is hard to do. It is also something that I don't really trust most authors to do, although in retrospect I think Disch did a fine job of it.
The biggest problem was that I really disliked the main character. The entire book is his journal, and is being read as he writes it. Very similar to Agyar, but with a totally different feel - in this case the writer is a professional poet, writing in the journal is required of him, and he isn't the main character of the story in the same way that Jack is the main character in Agyar - he is meant to be writing about the other people in the prison, the actual subjects of the experiment. There are often responses inserted by the people reading the journal, and in many cases he writes things deliberately to provoke a response. There is a large chunk of text in the middle of the book where resorts to writing gibberish, and I found that incredibly annoying. Once I got past the gibberish (and realized that it wasn't going to be like that for the entire book) I understood the point of it (the folks reading his journal had been even more irritated by it than I had been), I wanted to go back and see what sort of structure I could find in it...but was too irritated to do so.
I think my lack of enjoyment of this is largely due to the fact that I react very emotionally towards books, especially the first time, and this book is very black. Dystopias sort of have to be. But I really missed the point with this story. There's the really obvious 'twist' which comes as a big surprise to the narrator, but not really a surprise to the reader, but there is quite a lot more going on. Many of the characters (prisoners, experimental subjects) are actually super-intelligent and are busy hiding this fact from the folks in charge. And because the text you are reading is being written for consumption by the folks in charge, you get fooled right along with them. Which is really quite ingenious. Now I just need to get over my revulsion towards the setting and re-read it, because I suspect I will really enjoy it the second time around.
Editing to add: I just re-read it, and while it didn't make me nearly as grumpy reading it the second time around, I still didn't really enjoy it. While the story is clever, I still don't like any of the characters, and there is a grittyness to SF written in the 60s which depresses me. I've noticed this mostly in short stories, but many authors writing back then seemed to be assuming that the future was very bleak. That overpopulation was going to be a huge problem, nuclear war was inevitable, and that the Russians were the ultimate evil. Maybe it is really a problem of projecting forward and imagining yourself in this very strange world of the future. You would probably hate it if you were suddenly transplanted 100 years into the future (or the past), but as Sky pointed out to me years ago, most people manage to be quite happy, no matter what their circumstances. Humans are incredibly resilient. Maybe someday we'll manage to kill ourselves off, but in the meantime we seem to manage to enjoy ourselves. I think happiness is what's missing from this story. Granted, the characters are all stuck in prison (whether prisoners or guards, they're still stuck), but there are some genuinely uplifting moments, and some wonderful descriptions of food...and yet none of the characters ever seem happy.