This is definitely a work written with the lay church as the audience, which means that both the educated clergy and readers outside the church may not find much to draw them in.
Despite a rough start, wherein Schaeffer comes out with a very aggressive and unreflective tone, mishandling history, crouching at the door of fear-mongering, and glorying in intellectual bully tactics, The God Who is There finishes with a few far more moderate and thoughtful chapters, and appendices that continue in a less offensive, if not quite intellectually humble mode.
Schaeffer's argument in favor of Christian belief is primarily grounded in reducing complex theories into manageable villains. He knocks down Hegelian dialectics, Kierkegaardian Existentialism and Heideggerian Epistemology with relish, but his counterarguments are weakened because they are not well nuanced and appear to be mostly a form of dismissal. Greater detail in his analysis and a better sense of the role that citation can play in establishing case studies as representative of general trends would make the reader less dependent on blindly accepting him as an authority in handling very heady topics in order to be persuaded by his argument. The acknowledgment that he is not a philosopher and that he very well might be wrong should have been made in the first chapter, not put out as an appendix that borders on retraction. Additionally, drawing in the work of other thinkers with whom Schaeffer can agree would save him from painting his work as the single light in a dying world, something he certainly is not.
Once the argument is done with critique and moves on to dealing with Schaeffer's specialty, pastoral counseling, it improves immeasurably. While, in the text itself, Schaeffer still makes little room for personal humility, he does write with greater deftness and insight. His primary focus becomes the need for each person to come to grips with the realities around them, and in themselves, in a way that pushes them out of the fantasies and philosophical cages that imprison them. There are things which are really real, he says, and we are tricking ourselves if we do not face them. Enter the title of the work.