The book is a retelling of the story of Jesus Christ, however, with the revision that Jesus and Christ were two different people - twin brothers with decidedly different approaches to the question of what is Good. I found the title to be a bit of a misnomer; I didn't think Christ was depicted as a scoundrel at all, just someone with good intentions and poor judgement. I wondered as I read GMJ whether Pullman had come up with the title of the book first, then wrote it, realized it didn't quite match his original vision, but didn't want to part with such a juicy title. I don't know that this is so, but I imagine that it might be. Regardless, both Jesus and Christ were surprisingly nuanced and, I thought, sympathetic.
One of my favourite quotes from the book, from the chapter "Jesus In The Garden Of Gesthemene," page 197: "As soon as men who believe they're doing God's will get hold of power, whether it's in a household or a village or in Jerusalem or in Rome itself, the devil enters into them." Power corrupts, and absolute power corrupts absolutely; what power is greater than believing you're doing the work of God? Doing good in the world requires humility, and is not reconcilable with ostentation or pride.
Apart from the splitting of Jesus into two people, I don't know how consistent GMJ is with the gospels. I wasn't raised with any sort of religion, and for better or for worse, most of my knowledge of Jesus comes from pop culture depictions. In fact, if my best friend in high school hadn't convinced the bus driver on the way back from our senior class trip to play the Jesus Christ Superstar soundtrack, thus piquing my interest, I don't know if I'd know much of anything about Jesus. Ha! Take that, religious right! Most of my knowledge of Christianity comes from show tunes, that ever present staple of gay culture.
Overall, I give the book a thumb's up; it's not as scandalous as the title suggests, and I think Pullman does a good job of retelling the story and calling into question the more dubious aspects of Christianity (abuse of power, treating followers as sheep, &c.) while keeping the core values of the protagonist(s) intact. But if you really want to have your mind blown by Pullman's philosophy on religion, do yourself a favour and read the His Dark Materials trilogy.