I'll be upfront, the animation is sometimes very annoying in the first 'arc' of the series.  Mainly because the changes Youko goes through make you think you're losing your mind.  You're not.  Honestly.  There's a long involved explanation in the novel series about why she undergoes the physical transformation as the story progresses, but in the anime its simply explained away as 'Oh yeah you're a Taika so that's to be expected'.

Please be aware you're going to hear a lot of strange, odd and curiously similar sounding words while watching the series (whether in Japanese or English) and trust me just follow the context since they aren't always given enough explanation.  The English adaptation in particular seemed intent on tossing new words out and then giving us more new words that mean almost the same thing, sound almost the same but aren't really the same.  Its fairly easier to watch in Japanese with subtitles, but I'll get into that later on.

That all aside there's a lot to recommend Juuni Kokuki (or The Twelve Kingdoms as its commonly known as domestically).  Even without knowledge from the book about this complex world, a viewer can get a good feel for it from the anime.  Though Youko is used as the 'reference' point for each of the arcs (meaning she is the impetus behind why we're suddenly thrust into a 6-chapter arc about the Taiho of Tai and why he's missing), she is not the main character throughout the entire series. 

The anime covers Youko's ascension to the throne, the first part of Taiki's story (his disappearance from Japan, finding his 'King' and his present circumstances), King En and Enki's first meeting/history, the continuing issues in the Kingdom of Shou, and a few more side arcs that were based on shorter stories in the universe.  Aside from a few character insertions (Asano doesn't exist in the books), and Youko's larger presence throughout, the anime stays pretty true to the novel series. 

 Not sequential order of publication of the novels, but then the novels weren't published in the order of the events anyhow (technically 'Demonic Child' is the first book in the series, but largely covers Taiki's disappearance and wasn't officially part of the series until later in its run).

As I mentioned earlier they toss out words like 'taika', 'Taiho', 'hanjyu', 'shitsudou' and many many more quite commonly and the first dozen or so times it can be really annoying to listen to in English. 

I'm not sure if its because the voice actors don't give much life to the characters, or if they chose the wrong ones, but the English voical cast was some of the worst I've heard in a long time.  And its not like these were no names--I love the cast in various other animes, but here...I don't know.  It doesn't feel right.  Instead I tend to watch the show in Japanese with subtitles.  The words flow much better too--probably because they have a distinctly Eastern pronunciation, which the English VA's could not agree upon.

I believe this is available on Netflix, though someone correct me if I'm wrong.  If not, you can find the series for fairly cheap online and they've recently begun re-releasing the series on blu-ray (though I can't speak to the quality of it as I'm watching older DVD editions).