After all the negative comments I saw, I started with low expectations, but the book is really good. I enjoyed both the math discussion (including the footnotes :) ) and the autobiographical plot. I am not a mathematician, but apart from the very last chapter (which is in physics, a discipline I will never understand), I found it very well written and comprehensible. The story was very interesting, too. I would not recommend it to people who do not like math, but those of us, who use math, even though they are not mathematicians would tend to enjoy it.
Even if you still don't care about math after reading this book, his story of becoming a mathematician is captivating. The math is not trivial, but Frenkel explains very well the ideas without overwhelming the reader with technicalities.