Gundam 00 was the first long Gundam series that was split into two halves of about 25 episodes, rather than continuing for a full run of 50. In te end, it turned out into a decent series: it does the job it’s supposed to do, but it wasn’t anything really outstanding. The second season unfortunately was a bit weaker than the first.
While the first series had some interesting politics in the beginning, the second season drops the politics aspect completely and instead focuses much more on delivering solid action and the story becomes much more character-focused. Central to the story are various different conflicts between opposing sides, and these really range from utterly abysmal (Saji and Louise whine way too much throughout the series) to engaging and pretty good (Allelujahh and Marie). Especially Setsuna deserves credit, as he grows into a solid and capable male lead.
This approach has its good and bad sides. As I said: whenever the good characters take up the spotlight, the series becomes pretty enjoyable, but in the end the second season is just too formulaic. There are too many random battles that don’t stand apart from each other; they hardly ever resolve something and usually end up with both parties simply retreating. Most of the major villains lack development (especially the innovators, who apart from Regetta have as much personality as a paper bag), and all in all, this second season could have also been done in 13 episodes without losing much of the content or impact.
So in the end, this really is a series for those who want lots of solid action. If you’re not into that, then there really isn’t a lot left to watch in this series. The animation pretty good overall, but for me it was hardly ever really aestetically pleasing; the soundtrack is excellent, though. It’s not a bad series by far, and it has some pretty enjoyable parts (my personal favourite being the fall of the elevator), but there are lots of better mecha-series out there.