To start off, Wide Sargasso Sea can be difficult to read. It's written in a very conversational, almost stream-of-consciousness style from, remember, the mind of a supposedly crazy woman. This language reflects well the inner turmoil of her mind and life yet may make it difficult to become absorbed in the novel. The narration also changes halfway through the novel to her husband, Rochester, and back again without notice either time. Further sources of confusion can be discussions of race and class: it can be very difficult to remember who's who (black, white, mixed, and/or whatever offensive term is used to describe them) and why they dislike each other. All I'm really saying is that this book requires your attention and active participation, which is not unreasonable.
Whereas Bronte made a cliche out of her madwoman in the attic - Bertha Mason is everything Jane Eyre is not, the "bad" to her "good," - Rhys brings her to life, dissolving the stereotype. Bertha Mason is not just a loose woman who married for money, embarrassed her husband, and eventually lost her mind. In fact, she is very few of these things. Rhys rechristens her Antoinette Cosway, a child born into a broken home who experiences first-hand trauma that very few of us can relate to, classism (she and her family are called "white niggers"), upheaval, and grows into a woman who is used to satisfy men's greed. She has the roots for insanity, but also the roots for a sad, lonely life which she is forced to experience in Rochester's attic.
What Rhys does is humanize Bronte's characters. Antoinette was described unforgivingly by the plain Jane Eyre as "the foul German spectre - the vampyre," with a purple face, swollen lips, and bloodshot eyes. Rhys makes a person out of this monster who feels the pain of her brother's death, being disowned by her mother, cheated on by her husband, and tortured by neighbors. Yet this book does more than humanize the madwoman - it humanizes the man who locks her up as well. Rochester is an obvious villain; despite the innocent Jane Eyre's acceptance and forgiveness of him, he is guilty of locking a woman away, driving her to murder and suicide, and (in Jane Eyre) attempting to marry another woman despite his extant marriage. Yet Wide Sargasso Sea show his vulnerability in his efforts to please his father and succumbing to the trickery of his own family, who compel him to marry Antoinette for her money. He is flawed, yes, but human as well.
Wide Sargasso Sea is not the easiest of reads and probably not worth the effort if you haven't already read Jane Eyre. Yet if you're willing to read actively and reshape or at least reconsider your preconceptions of its principal characters, it is well worth the effort.
PS. It is unforgivable that it has taken me this long to post this book review as I read the book more than a month ago, but let's just say that book reviews are harder to write than you'd think. What made this one even harder is that I finished the book more than a month ago. I've done my best and if I missed a major point, I apologize. Hopefully I'll do a better job on the two other books I have awaiting review.