This series is so good that when it ends at episode 45 you'll find yourself wishing for more, I know most who have seen it feel like this.
It starts off innocently with Yoko being accosted at school by a handsome man with long blonde hair who wants to swear allegiance to her. Before she knows it, monsters?!? are attacking her, she's being defended by Keiki, she won't leave her friends and suddenly Yoko and the 2 friends she won't leave have been sucked into another world. Thus begins our journey into the magical land of the 12 kingdoms.
I admit, after the dramatic beginning, I was expecting this to be like the Escaflowne/El-Hazard type of series. That impression was swept away in under 10 minutes. Escaflowne is a good anime, but 12 Kingdoms is a Great anime.
What sets 12 Kingdoms apart is the detailed explanation of the political, social, economic and philosophical aspects of the kingdom. As the episodes progress, you learn how the kingdoms are organized, why they are structured the way they are, and the good and bad things about each type of government. I've never heard it explained better in any other anime.
The story and characters really suck you in. The more they reveal, the more you want to know and you're eagerly looking forward to each episode to see how the character's will react, secretly hoping for some of them to "grow up", others to "snap out of it", and actually even hoping for a few to "just die". At the beginning, I really wanted to slap Yoko a few times while shouting "deal with it". But by the end of the series Yoko has become the heroine that people identify with and they root for her. There are pivotal scenes that make you stand up and cheer in joy.
There aren't any extraneous "filler" secondary characters. While the motivations of some aren't revealed till much later, for the most part you can understand and sympathize with the "good" secondary characters and grow to hate the "bad guys".
The fantasy world created by the writer is capable of much more exploration via this anime. The plot has many other directions it can go in. Even though the series ended at 45 episodes, I know a lot of us 12 Kingdoms fans would be really happy if they made another 45 episodes as there is still a ton of unused potential - including all the other "unexplored" kingdoms, not to mention the rest of the black kirin story.
Watch this great anime but be prepared to feel sad at the end because there isn't any more to watch! The only flaw it has is that it ended too early.