Graves' scholarship is somewhat lacking. Or, to be gracious, the discoveries made since this book's initial publication cast a dubious light on some of the finer details of his argument.
That there are parallels between the Christ story and the stories of other religions' messiah figures is undeniable. And I'd tend to agree this proves that Jesus was one of many "sons of God" wandering the earth in his time period, and that the tale presented in the Bible mixed fact, fiction, and accounts from other places.
But there are more accurate books detailing this process. This book's worth is largely in its historical significance.