First, let me say that Dumas is one of my favorite writers of all time. Among others, I've read unabridged versions of The Count of Monte Cristo, The Three Musketeers, and Twenty Years After. I also read one of his shorter novels, Georges. I thorougly enjoyed all of them, and would include the first 3 listed among the best works of fiction I have ever read (Georges is also good but not quite as good as the first 3 I listed). When I saw The Last Cavalier on Amazon, touted as a "lost work" (which it in fact is), I immediately pre-ordered it, and patiently endured delay after delay before the book was finally released. It was something like 8 months after I pre-ordered it before I actually recieved the book, and the wait only served to increase how much I looked forward to reading it - I very much wanted to enjoy this book. Alas, I was disappointed!
The Last Cavalier reads almost like 2 separate works that have been pasted together. The first half focuses on Napoleon and on George Cadoudal. This half of the book is peppered with a number of very interesting historical facts (many of which I was ignorant of), but it is a bit slow at times. The first half also introduces our hero, Hector de Saint-Hermine and his family, but the focus is not yet on him. The second half is the story of Hector (aka Rene), and reads more like a novel.
To me, the most glaring flaw of The Last Cavalier is that the various plotlines are not tied together into a cohesive story. It is a long book, and unlike Dumas' other works it reads like a LONG book! The Count of Monte Cristo was itself over 1,200 pages long, but it was such a page-turner that it didn't at any point feel cumbersome or too wordy. Not the case with The Last Cavalier - a number of the wordy digressions are, in fact, nothing more than wordy digressions that don't add to the story. You really do get the feeling that Dumas is being paid by the word (which he often was). It took me a long time (about 2 months!) to finish the thing simply because it just didn't have the draw to keep me reading for very long periods at any one time, I kept putting it down and even contemplated giving up on it a few times.
The character of Rene is not particularly believable and despite the fact that he has faced great hardship, he's not that easy to sympathize with. He's invincible and is able to perform feats of daring, strength, skill, and even gluttony to the point that it's absurd. He also lacks a worthy antagonist - early on (and in fact it's stated in the book jacket) Napoleon is the long-time nemesis of Hector's family, but ultimately nothing ever comes of that. Hector does face a number of minor antagonists throughout the book, but none stick around long enough for their characters to get fully flushed out.
While this is a flawed work in my opinion, there are a few redeeming qualities. For one, the writing is excellent. For another, while the overall story isn't tied together very well, there certainly are brief periods of brilliance (when George Cadoudal reveals himself in a farmhouse; the wedding scene; some of the tales regarding the Companions of Jehu). These periodic flashes of greatness kept me from giving up on the story, as I would tell myself "ah, this is where the story really starts to get good!" ... alas, that never really happened. Finally, as I mentioned before the novel is peppered with a number of very interesting historical tidbits, particularly relating to George Cadoudal, Napoleon, and Lord Nelson. I'm not 100% certain to the general historical accurary, but some of the things I quickly checked on Wikipedia (which I realize isn't the most reliable source itself, but hey it's quick and generally is accurate) indicate that the history is pretty accurate.
In summary - if you are a huge fan of Alexandre Dumas and want to read everything he's ever written (or get as close as you can), then you certainly should read this. If, however, that is not the case or you are new to Dumas, do yourself a favor and read an unabridged copy of either The Count of Monte Cristo or The Three Musketeers, as his genius really shines through in both of those works.
If Dumas had time to actually finish The Last Cavalier (and by complete it I don't just mean write the ending, I mean go back through it and edit and delete or change things, as he often did before publishing a novel), it's entirely possible that this could ultimately have been a great work as well. But as it stands, it reads as a flawed and incomplete story.