Perhaps this is a little unfair... but the parallels between the series are so widespead and so obvious that it's hard not to consider it somewhere between a homage and bald-faced plagiarism.
A teenage boy (Kamina Ayato), who has only a distant relationship with his single parent, learns that she has prepared him from birth to pilot a GiantFightingRobot (RahXephon) for their SecretGovernmentOrganisation? that protects Tokyo from terrible Invaders. Before too long he's fighting a succession of GiantNotActuallyARobot?s, spouting lines like "I don't want to fight!", living together with two female members of the organisation (a tomboy, and an older woman), and becoming progressively more neurotic. Throw in the obligatory socially isolated, quiet girl with unlikely hair colour and mysterious abilities, and at the end, it all becomes terribly metaphysical.
But all that said... it's not NeonGenesisEvangelion. In spite of the parallels, the two series are different stories being told in different styles, and while RahXephon lacks HideakiAnno?'s messed-up directing talents and characterization, it makes up for it with a strong, coherent story and likeable, interesting characters. There are also enough well-thought-out twists in the tale to retain the interest of even the most cynical of this-is-just-like-Eva naysayers.
Even though it is just like Eva.
I'll say. There is a strong element of deliberate parody, I think. Example: Episode 19ish, where Shinj... erm... Kamina is sitting in the cinema. I just felt like jumping up and down and screaming 'we've seen that bit already'. Generally good, though - although an episode's worth of explanation short at the end. And, unlike Eva, you almost never want to strangle the protagonist. -- TI