Incredibly complex and full of politics and intrigue. The story is a lot like a Chinese soap opera -- long, angsty, romantic, with many characters and settings. However, the series can seem to drag for impatient readers or readers who don't like angst. The idea of "fate" figures heavily into this story.
However, the author has done a masterful job interweaving the stories of the various characters, mixing the past, future and present. The length of the series makes this story into a great epic which is much more worthy than any soap opera. Yes, it's often over-dramatic, emotional, and too tragic to bear -- but the beautiful art, the complex plot, and the strong characters all stand out. Definitely recommended, one of the best (if not the best) manhwa out there.
Finally, I'm surprised that some people complain about the art. There are so many characters that it is easy for some of them to look similar, but I've never been confused. If you look closely, the backgrounds, especially buildings, can be incredibly elaborate and detailed, more than any series I've ever seen. Even the costumes are well-researched and it's not even as if each character wears the same outfit often, as it happens in a lot of series. Although there are sometimes perspective flaws, Princess has the most attractive art that I've seen in a manhwa.