Well, I've just picked up this series, and I was pretty surprised and impressed with it so far. I think I was deterred at first because I'm not really into the "giant robot" anime genre, so when I hear robot my eyes usually glaze over and I click the back button. I was curious that this thread was getting so many posts though, so I gave it a try.

The way that the characters in each episode impact me emotionally reminds me a bit of Mushishi. All of them are so vividly different, and they're all a little bit heartbreaking in their own way. Poor Casshern, he's so confused, but he desperately wants to save them, and gets really down on himself when he can't. Everything about the world is so ruined and terrible, and yet it just adds to the beauty of this story. It reminds me of the quote from Kino's Journey: "The world is not beautiful, therefore it is." Some of those same themes are present here, I think.

As to the nature of the ruin, I'm very skeptical that there's anything natural about it. The minute any material is separated from a robot, it starts to crack and crumble. There's something about the 'consciousness' of the robots that slows it down, but once that is lost, they decay at abnormal speed. The advent of the ruin is also linked to a certain day, the day that Luna was killed. So, as far as I can tell, there was something unnatural unleashed upon the world that day that started it all. It might be a Matrix-type situation where the humans came up with the ruin as a weapon to fight against the robots. Maybe a bit far-fetched, but we don't know very much at the moment about the relationship between humans and robots.

Definitely going to keep following this one.