One of the strengths of the book is Upton Sinclair's passionate intensity about the need for workplace and housing reforms to give people at the bottom of the social ladder a fair chance to survive in this society. He convincingly shows throughout the book how Jurgis, Ona, and the other poor Lithuanian characters don't have a chance: the deck that is stacked against them. The reader feels their pain when they are cheated in the contract on the house they buy—a contract they don't understand—and the pain of them having no job security. It is painful to watch their hopes crushed. Sinclair's ability to paint a compelling portrait of immigrant struggles and defeats makes a strong argument for reforms to protect honest, hardworking people.

One weakness of the book is its sentimentality, especially in its portrayal of women. Modern audiences might not react well, for example, to the utter purity and helpless weakness of a character like Ona, which feeds into Victorian stereotypes of the angelic woman as "victim" and misrepresents the reality of who women are.

This answer is highly dependent on the reader.  I would say one strength is the way that Upton Sinclair is brutally honest about the horrific work conditions that existed at the time the book is taking place.  I remember reading the book.  It's been two decades since I read it, and I still remember the feelings of revulsion I had while reading the book.  The factory descriptions were so gross.  I still remember how sad I felt as Ona's quality of life declined as the book progressed.  You want so badly for things to turn around for her, but they just never do.  The book does an amazing job of tapping into reader's emotions.  

Related to the strength of the book's brutal honesty is a weakness of the book too.  It's a dark and depressing book.  It ends on a real downer and doesn't leave the reader with much hope, optimism, or possible solutions.  Sure, that might be more realistic, but hopeless readers aren't usually readers that are inspired to action.