Birth order psychoanalysis is something I tend to rank just above astrology. However, as a firstborn myself, I was curious enough based on this book's title to pick it up and peruse the table of contents. The content in this book is spread so thinly and repetitively that it's the ideal book for skimming. You can consume the whole thing in a few sittings.
I finished the book with an elevated opinion of the whole birth order thing. It seems that it's not exclusively about your chronological order, but really more about the kind of relationship you have with your parents. (The stickler in me wishes they'd then rename the types to reflect this.) That said, I personally fit the firstborn profile to a T: seeing the world in black and white, reliable, organized, leader, list maker, conscientious, creature of habit, achiever. And some of the advice for using your "advantage" seems quite useful: pursue excellence rather than perfection, let some things go undone, wait before offering your opinion, learn to bounce back when you fail, stick to your guns, etc. But those two sentences pretty much sum up the content of the book; the rest is mainly anecdotes about hypothetical, stereotypical firstborns, which I guess you are supposed to identify with, but to me they seemed too extreme/fake. The exception is Chapter 6, about the "Critical Eye", and its treatment of parent-child relationships, especially when one or both have firstborn personalities. If I had children myself, I'd want to re-read this chapter regularly as I think it has good advice for curbing some of my natural counter-productive tendencies.