I watched what I did from my perspective as a fan of the game, and I was somewhat disappointed, but also confused. The series loosely follows the plot of the game, but not exactly. Characters are introduced too soon, for example, such as Albedo, Ziggy, and MOMO, or eliminated, such as Commander Chekhov, leaving me wondering how the series would later fill the newly empty places in the plot.
The series' approach, which conveys information in relatively large chunks compared to the tiny pieces delivered by the game, largely ruins the mystery. Also, the menace of the Gnosis seems less extreme. While it hardly felt more severe at the corresponding point in the game, I wonder how the creators of the anime made the Gnosis matter without Chekhov, assuming they did at all. There also seems to be an emphasis on bringing the characters together, rather than laying out a little of their backstories first, and the series starts off much faster than the game.
On to the good stuff. In the case of Xenosaga the Animation, the good stuff is the combat, where KOS-MOS still kicks at least nine kinds of ass, but probably more, and where Ziggy still seems ineffective by comparison, and Lieutenant Virgil is the only other one who has yet fought at all. The intro animation, too, is great - I just love KOS-MOS with angel wings. The music isn't bad either, and after a nearly musicless game, having theme songs is quite the relief.
The animation, however, is weird, to say the least. Eyes seem odd, with a number of rings inside them, and certain characters, including Albedo and Allen, seem oddly proportioned, even blocky. KOS-MOS still looks cool, though, and the characters are all recognisable.
All in all, I wouldn't recommend the series, especially if you are attached to the game. It's still passable, and mildly entertaining, but not enough to hold my interest.
This series has not, as of the date of this posting, been licensed, and this review is based on the Anime-Kraze translation.