This is one of those animes you can't just watch once. Even after three viewings, there are still elements of the plot that slip by my grasp.
You can tell by the dialouge that a lot of thought went into planning the world. Everything happens for a reason, and there's science behind all of it.
This show proves that a lot can be done in 26 episodes, something I wish American television shows could pick up on. The show's multitude of characters are fleshed out, plots grow and weave together as events begin to build towards a finale.
The clues are all there for the viewer to pick up on, but with the science, this may be difficult at first. But when the show finally reveals its secrets, it's easy to go back and say "Ah...that's why this happened!"
This became the first Anime DVD set I bought, and I was glad to do so. These are the kinds of Anime I wish G4 would show more of.