Acquisition Method: Up until recently, it was only available fansubbed. However, it recently got licenced by ADV Films so it should be available on DVD soon.
Format: It's a series of twenty-six episodes, all of which have been broadcast in Japan. That's good to hear, because once you get your hands on it, you'll need to watch the next episode immediately to find the answers to the questions raised in the ep you just watched. Every episode answers another question and drops another bombshell that needs an explanation.
Genre: It's an atmospheric post-apocalyptic occult sci-fi mindscrew, and probably a few other genre could be thrown in there as well! A sense of melancholy and darkness permeates the whole series, so you may well get the impression that the ending will be downbeat.
Plot: A world catastrophe occurred in the year 1010, an event later dubbed "Twin X". The "sheltering sky" lies over the entire world now, and stops all computers and technology from functioning. The world appears to be decaying.
In this world, two children, a boy and a girl are being chased by debt collectors, but they are rescued and taken in by a group who refer to themselves as Gilgamesh. Yet another group retrieve them and take them to meet the Countess, who pays off the debt collectors and therefore buys them.
The boy, Tatsuya, was born after "Twin X" and so is able to use Dynamis, a kind of energy that allows the user some forms of psychic powers. He joins with the other Dynamis users that the Countess has taken in; Tsukioka Tooru, Fujisaki Isamu and Omuro Fuuko.
There are many questions to be answered still. Who is the children's father, and how did their mother acquire such massive debts? What are the origins of both Gilgamesh and the Countess' group? What links them all?
Seiyuu: Shibuya Shigeru (Gene Starwind in Outlaw Star) voiced Novem, one of my favourite characters in this. I also really liked Itou Kentarou (Geisel in Weiß Kreuz Glühen) who played Fujisaki. Kazamatsuri, a character I thoroughly hated (and was meant to!), was voiced by Fujiwara Keiji (Takatori Masafumi in Weiß and Enjouji in Kizuna). Saiga Mitsuki plays the role of the Countess. She voiced Wolfram in Kyou Kara Maou and Jing in King of Bandits Jing, so this only goes to prove how versatile she is!
Also making brief appearances are Nojima Kenji (Toudou in Weiß Kreuz Glühen, Setsuna in Angel Sanctuary OAV) and Suwabe Jun'ichi (Atobe in Tennis no Oujisama).
All in all, an excellent cast!
Technical details: Admittedly, in some places the translation was patchy (especially the OP and ED themes), but I fully expect this will be fixed for the official version when they have the song lyrics in front of them.
Most people who didn't like Gilgamesh complained about the animation because the characters are not drawn in a 'typical' anime style. It's very stylised, and some people couldn't get past that. I think they're missing out! In any case, the plot is the important part, and what will keep you hooked.
Music: I'd put this in the haunting instrumentals category. I certainly wanted the soundtrack afterwards!
Squee factor: This has the one of my favourite ingredients for anime: a sadistic attitude to plot revelations. I believe I referred to it as "malevolent inevitability" when I first saw the scene in which Tatsuya's parentage is revealed. Best of all, would be the frequent feeling of "I don't believe that just happened!"
Another great feature? One of the many subplots in the show involves romantic hints towards a yaoi relationship. Sadly, like most of the coolest parts in this, just saying who it was between or further details would constitute major spoilers.
Fans: You can find the LJ group for the series here: twinxx.