RahXephon redefines many parts of anime genre. It has amazing characters and the mecha are rather interesting and not your usual crash-and-bash toys. And, a majority of the conflicts take place within the characters themselves and not with the mecha, which made it rather engrossing. You can only care so much about what happens to a machine. But the characters?
Also, whenever Sou-chan stops smiling; fear. And no, I'm not talking about THAT Smiling Sou-chan.
Over all, it's weird, fucked up, and has more twists than even Infinite Ryvius (which I was sure took the cake on plot/character twists). I really enjoyed all of it, and I hope that it is successful when it makes it to the US. I know it's been liscenced, and there's talk of putting it on that supposed Anime Cable Channel. And when it's released here, I'm getting it. And there's a movie in production too! Man, I can't wait. Must see more!! Though the ending, after the fact, left me VERY satisfied.