The book begins with Lewis presenting the existence of God as a very reasonable consideration. I believe Lewis' past as an atheist and the path to his conversion helped very much in this regard. He makes a very strong case on behalf of the existence of God, and he does it in such a way that any thinking person reading the book would have to admit that the arguments are pretty convincing.
As the book continues on, despite his best efforts to avoid it, Lewis' Anglican faith begins to make appearances here and there. There are also times that, while it is obvious that Lewis' was a brilliant man who I admire greatly, he gets in, seemingly, over his head. There are many doctrinal issues that are presented and supported by reason and logic when the most reasonable approach would be to simply rely on revealed truth. I know that in the beginning of the book the purpose seems to be that Lewis' was trying to make a plausible case for God without Scripture so as to cause the skeptic to see that there is sense to the idea. However, when dealing with particular doctrinal issues later on in the book, while his approach would retain the skeptic it loses the biblically grounded believer.
Overall this is a very good read and an easy read at that. The chapters are short, if wordy. His conversational tone is easy to follow. And again the book makes you think.