I read the chapters in the manga a long time ago, and I finally was able to see the animated version.
I did have to dock a point or two here and there; mostly because there wasn't any explanation leading into the entire story. Anyone who has read the manga knows why Nataku was ordered to kill Goku in the beginning, but that was never explained in the Gaiden animation.
Otherwise, it was excellent.
Story
I did dock points off the story, only because the first episode wasn't told in a very coherent way. There was little to nothing which led up to the major "crisis", or explanation on why Nataku was ordered to kill Goku. The flashbacks which randomly occurred sometimes actually interfered with the flow of the story itself (which caused me to dock a point off of animation, as well).
Without giving anything away, the plot revolves around the revolution/revolt which took place in the Heavens 500 years before the start of the series, and the chaos which surrounded it.
It all started off with Nataku being ordered to kill Goku. This wasn't explained in the anime itself, but for anyone who watches this and gets confused, the order was given because Goku was deemed a dangerous existance to Nataku (as his "father", the organizer of all the chaos, Li Touten, wished to keep Nataku as his puppet).
Tenpou, Kenren, and Konzen attempt to defend Goku (even after his diadem is destroyed for the first time in Heaven and he reverts to his Seiten Taisei persona), which leads to them being declared as traitors and heretics of Heaven.
While they attempt to run from those who would have them killed and try to make it to the Lower World (Earth) in the process, Li Touten uses their rebellion to further his own ambitions.
Characters
Any Saiyuki fan should know by now, but all of the protagonists in Saiyuki (with the exception of Goku, who was imprisoned) are the reincarnations of their souls who used to reside in Heaven. Some of these scenes were referred to during the final arc in the Saiyuki anime.
Konzen is the nephew of Kanzeon Bosatsu (the Merciful Goddess), and is put in charge of a heretic who was borne from the Earth (Goku). He was a typical paper pusher in Heaven who didn't really care what went on with the higher-ups, or know what it really meant to "live".
Goku is much like how he was before he was imprisoned, only without the huge appetite he has in the series. He, himself, didn't know of his true form, and was brough to Heaven and put under the care of Konzen (whom he refers to as his "sun"). He also makes friends with two of the army members, Tenpou and Kenren.
Tenpou is the Marshal of the West Army, and though he does have the same sense of humor we see his reincarnation have, he isn't as neat or organized; he also smokes. However, he's a brilliant strategist and someone his men looks up to. Even when he was branded a heretic and traitor in Heaven, his platoon of 14 men still followed him.
Kenren isn't as brash as his reincarnation, though he does show some of those traits. He has a sharp mind, and a slightly sarcastic personality. He's pretty much the comic relief at times, but those times don't occur very often. He also surprisingly has a very strong sense of work ethic.
Goujin, the Dragon King of the Western Sea, is the commander of the West army, and the superior to Tenpou and Kenren. Most of the narration is done in his voice, as he's recalling the events which happened. He was used as a hostage when Konzen, Tenpou, and Kenren fled with the unconscious Seiten Taisei, and though he doesn't really show it, he obviously cares for his subordinates. Though it's not mentioned much in Gaiden (as in, at all), he was reincarnated with the other three who died (as Hakuryu).
Li Touten is the main antagonist, who attempts to overthrow the Jade Emperor and Heaven to rule it with his "puppet", Nataku, who was created for the sole purpose to be the War God (the title granted to the only one who is actually allowed to kill, though apparently, the army is also allowed to kill those branded as heretics and traitors in Heaven).
Overall
I would have preferred an additional episode to help build into what led up to the order of Goku's death (which spurred the entire thing on), but otherwise, it followed the manga perfectly without going back over many of the scenes we already saw during Homura's arc in the anime.
It also would have been nice if the episodes flowed a little better rather than include flashbacks to scenes we've already seen. I know it was an attempt to build tension or an emotional scene, but it failed and only interrupted the plot.
On the plus side, they didn't need to reintroduce the characters or who they were, because anyone who watches Gaiden would already know that (between the manga and the final arc of the first season of the anime).
There is a lot of blood (a heck of a lot more than in any of the seasons of the anime; in fact, probably more than all of the seasons combined, and that's including the Kami-sama arc), and one slightly gory scene (though it's only for a split second). Just fair warning to anyone who's bothered by a lot of blood.
I can't give this a full 10 out of 10 due to the lacking beginning and the calm scenes used in the middle of the action, but it's still a must-watch for fans of the series.