I'm reviewing, here, the entirety of O'Brian's Aubrey/Maturin series, because I consider it to be essentially one novel.
The first, and most astonishing, strength of this series is in its characterization. Not only are the contrasting, yet inseparable, friends Jack and Stephen believable, appealing, vividly human characters, but they change realistically through time. To the reader, they appear as "real" people with "real" lives, perhaps more so than some of the the flesh and blood ephemera around us. The secondary characters, too, shine -- Killick is priceless.
Research, of course, is O'Brian's other great strength. It's not only the ships, about which he seems to know everything. There's no aspect of the period -- food, dialect, religion, music -- in which he does not seem to be well versed. And he conveys this information to the reader in interesting ways, rather than encumbering the text with massive info-dumps.
One often overlooked bright spot in this series is its humor. Too often historical fiction has a self-consciously grim quality. O'Brian can be grim -- crushingly depressing, in fact --but... "Swiving Monachorum".
Action and battles are not, strangely, this series' strongest point. When we get them, they're great, but too often they are skipped over or told in a distant third-person viewpoint. But the worst here is still very good indeed.
I would recommend reading all of these, in order, starting with the first one, right away, as soon as you possibly can. It's true that The Hundred Days marks a low point -- I agree with the reviewers who cite O'Brian's loss of his wife as the reason -- but Blue at the Mizzen, under which I've posted this review, marks a triumphant return of the author's powers.