If you're a Dumas fanatic like me, you'll probably want to read this book just for closure, regardless of what I say! But I'll write a review anyway. The problem with a novel like this is that I don't know what flaws are attributable to Dumas and the possibility that he was slipping, or cutting corners, as he got older, and what's attributable to the person who finished writing the novel.
It could have been much better if it were written as two separate volumes: one, the general history of the Napoleonic era which is presented in the book, and the other, the history of the Comte de Sainte-Hermine. So much of this very large book has nothing at all to do with the titular character. In fact we are well into the book before the man ever shows up. Then in a chaperoned tete-a-tete with the woman he loves, he divulges the entire history of the Sainte-Hermine family to date. (So we don't learn about his previous history as it's happening, as with Edmond Dantes in "The Count of Monte Cristo"; we're simply given several pages of Sainte-Hermine hitting the highlights for his intended. They become engaged, and at the betrothal dinner he mysteriously vanishes before signing the wedding contract.
Then we have another huge section about Napoleon, the Royalist rebels, etc. A very long section! It was a very GOOD section but I'd totally forgotten about Sainte-Hermine when suddenly we learn he is in prison and begging Fouche to execute him rather than keep him a prisoner. This brief scene takes a few pages...then it's back to a whole big, big section about Napoleon and his troubles. It made me wonder why this book was titled after Sainte-Hermine, since up to about the midpoint of the book, he's a completely minor character...almost a glorified extra.
At the approximate middle of the book, however, the Comte gets out of prison (legally) and the narrative switches to actually being about his life as he is living it. From here to the end it's mostly a very entertaining story of Sainte-Hermine and what's happening in his life, with a few sprinkles of the regular history in the background. This is how I expected the book to be from the start. So it sort of evened out in the middle and got better as it went along.
This also suffers from comparisons to the similar Monte Cristo. In the latter, we know that Edmond has spent his jail time learning from the Abbe Faria and then that he spent the next X years undercover, learning things to create his Monte Cristo persona. Sainte-Hermine, by comparison, spends three years in prison, during which we are told that his hobby is reading. Afterwards, though, he comes directly out of prison and into the narrative, where he shows himself to be an expert at just about everything, including (!) chugging three bottles of champagne that have been poured into a big bowl, and showing no ill effects. Don't you thimk a man just out of a 3-year prison stint would have some difficulty holding his liquor?
So, as a Dumas fanatic I'm glad I read this, but I'd have to rank it absolute last on the Dumas list. If he had stuck to a plain historical novel of the time of Napoleon, then, well, it would probably still be last on the list, but not by as wide a margin.