I found O'Brian to be absolutely addicting, and, unlike some of the other posters, I never found it to be slow going/hard reading. The humor is subtle, but wonderful. I love the endless interactions between the captain and the doctor, the absolute mistrust the crew has in regards to the doctor's incompetence in the sea. I found myself rooting for the captain as his fortunes rose and fell. I absolutely adored the characters. The story lines were also always very interesting, but this series is definitely character driven.
I find reading history a bit dry, so this was a wonderful way for me to imagine what being in the English Navy was like. The brutal discipline and work conditions, the opportunity for real riches, the barbaric nature of the combat, the bonds formed and broken among the men, the angst and fear of their wives who are left behind for years at a stretch, etc. It all lives inside of me now.
I found some of the later books a bit weak - I remember waiting anxiously for #20 to come out, and being not terribly happy with it. The earlier books are just surperb in my estimation. They stand as literature, not just as sea yarns.
i've never read Hornblower, so I cannot compare these to him. I think I shall rectify that....
roger