I like Penn Jillette. I have always found him entertaining and I should say off the top that I found the book entertaining but you will find no spiritual epiphany from reading the book (unless perhaps this is the only book you've read in years and in that case maybe you should read more). This book is just mind candy.
Penn's argument for atheism comes down to if you answer "I don't know" when it comes to the question "Is there a God" then you are an atheist. Joseph Campbell found that certain mythological beliefs are universal, archaeologists have proven that Jesus lived and walked the earth, Paul of Tarsus had some sort of conversion on the road to Damascus and Albert Einstein found the evidence of God in the order of the universe. If you asked Joseph Campbell or Albert Einstein if there was a God, their reply of "I don't know" certainly wouldn't indicate that they were closet atheists. Far from it and they are much more brilliant people than Penn Jillette.
God, No! is one of those books that famous people write when their business manager gets them a book deal because they happen to be famous. Sometimes the famous person has a ghost writer spill out the words but this is not the case with God, No! You cannot help but read the book with Jillette's voice ringing in your head. The stories he tells are amusing and sometimes very touching.
I recommend the book but just don't think you are going to have your life changed by reading it.