Post Captain has far fewer battles and intricate sequences at sea than Master and Commander. Despite my ensuing ability to follow the action much better, I didn't like the book as well. I felt quite sorry for Jack Aubrey through most of the novel, mostly because he's so unhappy during it. Stephen Maturin is unhappy for a lot of it too, but honestly I expect Maturin to be downcast and Aubrey to be happy. An unhappy Aubrey is like a miserable puppy. It's just wrong, and one cannot blame the puppy for it: you have to blame the people who keep kicking him. (In one of the delightful ironies of O'Brian's work, the really irritating people are never the ones on the other side of the war. Jack gets on fine with his enemies: they have a straightforward relationship of trying to kill one another until one surrenders, and after surrender they're quite civil. No, it's the allies one has to watch out for.) Near the end of the book, Maturin notes that Aubrey has grown up and is no longer as cheerful as he once was. I'll grant that Aubrey did need to mature, but I really hope he gets back to being upbeat and good-natured again in the later books.
One of the charms of the work is that O'Brian is quite indifferent to genre. The first hundred pages might as well've been an Austen novel except for the male viewpoint characters. The story gets actual significant female characters with speaking parts and everything: not necessarily great female characters -- I didn't think Villiers was very well drawn, but perhaps this is because the POV characters are unreliable narrators in regards to her -- but it's an interesting change.
Some of the non-sea action comes across as contrived and melodramatic, which didn't help my general impression.
Overall, I'll give it a 7, and I've started to read the sequel. Which the author starts off by proving he's a bastard. Really, Mr. O'Brian. You ought to be ashamed of yourself.
Note: comments may have spoilers, if I get to talking to alltoseek about it.