I made a new account just to comment on this series. It's quite good compared to others- it stands even without the sex. The sex here is not just for the characters to have fun. A way to put it is that this is a story about pirates that involves sex, NOT a story about sex that involves pirates.
Compared to other yaoi mangas' this one deserves a top rating for the fact that the plot isn't paper-thin. Mangas like Viewfinder are eye-candy with sex playing no particular part in plot development. I enjoyed the fact that the plot in this story is plausible and believable. One way to judge whether it is a stable plot is to remove the sex scenes and see whether the plot still makes sense. It works for this story.
A lot of people have commented that Canale cries too much. But Canale doesn't really cry all the time- he puts on a perpetually mournful face, yes. I think that it has a very good point- to emphasize that Canale is a weak, unstable character. You WANT to hate him because he's so weak. I think it's a great way to show how a character can be flawed. He's not supposed to be a brave character who's happy despite his sorrows- all his past sins have broken him into a depressed, deeply unhappy person unable to find solace in his lonely life. His relationship with Ayace builds a foundation for him to heal himself and his wounds.
Another reason I enjoyed this manga is because I found that it draws many parallels with another manga I loved, Tsubasa Reservoir Chronicles. Canale is somewhat like Fai and Ayace like Kurogane. Canale and Fai are both haunted by tragic pasts, i.e.
Canale's family hates him while Fai's family considered him a curse. Both of their families tried to kill them. They were both found and loved by kind men whom they were forced in a way to betray and the betrayal deeply wounded themselves more than it did the people they betrayed. They long for someone to love them. Both, for some reason, wish to die but are unable to take their own lives.
Ayace and Kurogane are both men who are warriors, brave and strong, sometimes appearing harsh. Both try and do repeatedly drag their respective partners (Canale and Fai) away from foolish acts that include self-harm. They refuse to allow them to die (Ayace refuses to let Canale take his life and does not leave him in the burning mansion while Kurogane refuses to allow Fai to die after his eye is stolen.) Their strong characters compensate for Canale's and Fai's weak ones.
In other words, Corsair is almost an X-rated and much shorter version of Tsubasa, minus Syaoran and Sakura.
Because this series is short, there is not much development to other characters but there is enough to make the story good. Aura is obviously not someone to step on while her twin is the eager young lad who is willing to accept the guidance of his elders. The villains of the story have enough backstory to make their motivations and ambitions plausible. The story is however mostly centered on Canale and his reluctantly growing love for Ayace, as well as his guilt for his past.
Whew, long review! Warnings for this story: some may consider it dubcon. Some may consider Canale to be a very weepy moody character. I think it's just right for a few hours of entertainment. It's not great literature, but it's a good little read.