I tried to give this a fair shot, I really did. Unfortunately, I have to agree with the majority of views here. It's difficult to enjoy the story completely because of the way Diana and the royal family are depicted. Even more so, perhaps, because the western world has it's own images and ideas of what Diana, Charles and the royal family are like, especially with what is portrayed in the media. It's so hard to take a story seriously, for example, when we compare 'Prince Charles' in the manga, who looks like a handsome white knight straight out of a romance novel, to the 'real life' prince Charles, who is... considerably less good looking. The overdramatisation in the story dosen't help either.
Honestly? for me, the manga would have been far better had they kept the story but made up the characters and changed the names, because I've read similar shoujo's where the characters are purely inventions of the mangaka's imagination, but enjoyed it alot more than I did this. To be fair, if you like dramatic and angsty shoujo's and can suspend your disbelief, I would say go for it. The art is pretty and easy on the eyes, plus the characters are not to bad, in their own angst-ridden way. On the other hand, if you are looking for an accurate portrayal of the story of Diana, I'd be more inclined to avoid this like the plague.