I'll start with a quick bit of background, in case anyone reading this has been on another planet for a while. This book is Heinlein's long-lost and forgotten first novel. After tuberculosis ended his Navy career, and after losing an election to the California State Assembly, he wrote this book. He submitted it to two publishers, who rejected it, and after that it stayed in the bottom of the drawer. A couple of years later, he sold "Lifeline" to John Campbell, beginning the writing career the world knows. Before his death, he destroyed his own copy of the manuscript. The fact that he'd written it was a minor footnote, until another copy of the manuscript was unearthed in the garage of someone who'd been a grad student working for someone doing a biography.
I'd heard a fair bit of discussion of this book before I read it, and I didn't have very high expectations, but I thought it would be interesting to see the seeds of his later work. The story was better than I'd been led to expect. Some people have said that it doesn't deserve to be called a novel at all, that's it's just a series of lectures held together by a very thin, contrived story. Although it's true that the plot is a bit contrived, and it does have long lecture passages, the story did hold my interest. Even this very early, unpolished Heinlein had a gift for stringing words together so that I want to read them -- Heinlein is another of my "grocery list" writers, and it's there even in this book.
Since probably about half of the book is lectures, it's important to note that they are interesting. The history is very dated (he failed to predict World War II, the bomb, and radar; his extrapolations past that just diverge farther from reality); the technology is odd (rockets for transportation around the world are commonplace, but in 2089 they're just getting serious about reaching the moon); the psychology and sociology is optimistic enough that you want to believe it, but it's tough; and the economic theory is quite interesting, although the concrete examples don't hold water.
Overall, an interesting experience, and a better book than I thought, though certainly not great. If you're a serious Heinlein fan, you do have to read it -- so take heart that it won't take that long and it is worth the time. I give it a 7 out of 10.