Mr. Bogle makes a solid argument repeatedly, by approaching the same question from multiple directions and supporting it with unimpeachable numbers. It reinforced some views that I had already developed on my own, but also clued me in to the adverse effects over time of high expense ratios and transaction costs.
When I finished the book I liquidated a stock that pays a decent dividend but seems to have little growth potential, and a mutual fund that has excessive costs that I had overlooked. I put the money in a low-cost S&P 500 index fund that already made up a significant part of my portfolio.
I agree with other reviewers who have criticized the book as repetitive, but it's worth reading for its style, wit, and clarity.