Summary: This book was not written in an attempt to convince the staunch non-believer. It was written for those who believe and those who doubt their non-belief. I felt that his opening chapters regarding the moral argument presented a strong point in a weak way.
Lewis used very little scripture in this book, but I do not see that as a weakness. If Christ genuinely is the Word, then his message should make perfect sense even apart from the written word.
One of the things Lewis demonstrated very well was the fact that if you look at man's dilemma as being fallen, and consider how he came to be in this dilemma, then the solution that Christianity offers makes perfect sense.
The concept of the trinity is also covered very well.
Finally, the chapter on God and Time sheds light on a few misconceptions about God's nature, and introduces a number of different ways of thinking about time that make it easier to see how God can hear everyone's prayers, or how the fact that God knows my future doesn't mean that I have no choice over the matter.