The description makes it sound pretty cliched, and I started reading very reluctantly; but after the first few chapters, the story started to become quite mysterious and intriguing.
The characters give the first impression of being rather stereotypical. There's the wonderful and responsible older brother (and crown prince), the polite and mellow but rather mediocre second brother, the fiery and rebellious (albeit good-natured) third brother (who also happens to be treated with contempt because he is the king's illegitimate child), the haughty and high-minded youngest sister, the scheming and unfaithful military and political advisors, the mysterious rescuer who arrives like a bolt from the blue to try and save the kingdom for no reason in particular...you get the idea. However, you would be foolish to give up on +C: Sword and Cornett because of these initial appearances. Very soon, most of the characters change for the better and become very interesting; their actions in later chapters are not in the least cliche. So long as you have patience, you will soon find yourself being drawn in to this manga.
The art isn't very good; it's rather commonplace and not pretty or elborate, but is clear enough that you can always figure out what's going on, even during fast-paced action sequences...so that's good enough for me.
Summing it up; at the end of Volume 5, the storyline remains gripping and has just begun gathering momentum. Hopefully, the conclusion will not disappoint.