RahXephon is a masterpiece. All the answers to all your questions are there if you look for them. And, the folks who made the show don't try to give you a brain fuck, sell you religion, or toys for that matter. They've also made other great anime as well.
As for the Movie, I would watch it. It gives you another angle to the story that's interesting. If you can get your hands on the Quon special that came out with PS2 game, there's some more interesting tid bits that add to the story.
Anyhow, Good Shit Bones! Good Shit!
And for you EvaFags, this is what Eva should've been! And, the producers already said they were paying some homage to Reideen for the Mecha and weaponry.