I think if you look at Guns of the South from a pulp fiction, sci-fi & fantasy point of view, it's an OK novel.
However, it's NOT really an alternative history/counterfactual scenario, and anyone who thinks otherwise is a dimwit. It's not a story of "what if?", but a story of freaking time travel! It's sci fi; a counterfactual novel is historical fiction. Compare anything Turtledove does to, oh, Robert Harris's Fatherland and you get the idea.
Finally, I must admit when I think about what kind of mind would produce a story about time traveling South African white supremacists trying to preserve a slave-owning Confederacy for their own ends ... well, let's just say I would not want to go get a beer with Turtledove. Ever.