It's a well known but really good book about a man with an IQ of 68 who is given an operation that turns him into a genius. It's full of ideas. I was surprised at rich in ideas the book is. It explores issues of identity, the ethics of changing people fundamentally through science and issues of how we treat people with mental retardation.
What I found most impressive about it was the characterization of Charlie as he grows more intelligent. His intellectual growth accelerates but not his emotional growth and he must struggle with both his past, his new situation and his sudden understanding of how people have treated him in the past.
That means the characterization and insight into human psychology is really good and that is normally the weak spot for science fiction novels. It's really good, The writing style is quite straight-forward.