Along with Gundam Wing and Dragonball Z, The Big O helped pave the way for the current anime explosion in the US. It combined Western noir-ish design (owing more than a little to the Batman animated series) and giant robots (courtesy of Giant Robo alumnus Keiichi Sato) into something very unique. The only problem with the show was that it ended after 13 episodes without any resolution.
This travesty has finally been rectified. A cooperative effort between Cartoon Network, Sunrise, and Bandai has brought us the second season of the Big O. All of the good things about the first season are back, along with a few extras. The plot really picks up in the second season, as Roger and company get closer to the truth of what really happened 40 years ago. The stand alone episodes of the first season are gone; almost every episode from season two ends with "To be continued." Also, the animation is much improved. There is a great deal of computer animation in the second season which seamlessly integrates with the traditional cell animation (which itself seems to have a wonderful digital sheen).
For returning fans: If you enjoyed the first season of the Big O, you will adore the second season. It's everything you could possibly want and more.
For other interested parties: If you liked the Batman animated series (the older episodes, not the new ones with the Justice League or Superman), or you though Cowboy Bebop would have been a lot better with some giant robots, then I can't recommend this show enough. Heck, even if you don't like those things I recommend the Big O. I suggest checking out Volume 1 of the Big O. There's a lot to digest here and you don't want to start in the middle.