Just how clever should a reviewer be? One answer would be "as clever as can be", someone able to see the hidden intricacies of every event. Another ideal would be to be as clever as the average anime fan, so you can give a review with the widest applicability. But in the end it comes down to "as clever as you are"... and you'll see why this is relevant shortly.
It's a normal morning in Tokyo for Ayato, a 17 year old schoolboy. Sure, his mother is a bit emotionally distant, but that's normal as well. Thus he is somewhat taken by surprise when, while on the train to school with some friends, a serious and intense war breaks out. All of a sudden there are tanks in the streets and fighter planes roaring over head. It seems that the world, which he had thought to be peaceful, is anything but. Even worse is the question of who is fighting, and over what. And it starts to become clear that he might not even be on the "good" side of this suddenly created front-line.
More importantly the world seems to have suddenly turned both weird and taken a distinct interest in him. All of a sudden there's rather intense people who want to have a quiet word with him, and are willing to take violent action against the other intense people with similar goals. Technology that shouldn't exist is seen in the sky, People and places turn out to be not what they appear to be, and even worse all sorts of odd, potently symbolic, things seem to be happening. This new life is dauntingly complex, and the stakes in this game climb ever higher. On the positive side he has found at least one friend on whom he can depend, and since that friend appears to be the most powerful mecha in existence he'll at least have some physical safety while he works out what the hell is going on and what part he will play.
Yes, it's a pretty "nothing" synopsis. The main reason is to avoid spoilers. The other reason is because without giving really "big" spoilers the synopsis would make even less sense than the sort of abstract one I wrote. The main thing is that it does contain a giant mecha, as prominently displayed on much of the artwork. But it's not at all a mecha show. It also contains a war between two factions in which Ayato is both a powerful force and inflicted with split loyalties, having ties to both sides. However it's not really a war movie, both of the sides are actually relatively passive after the first episode. What this means primarily is that if you wanted hot mecha action and great battle scenes your probably going to be disappointed. In this series the extremely odd looking mecha battle by "singing" at one another, which fits the story but is visually boring. Nor is there really any sense that Ayato is in much danger of defeat.
It is rather similar (perhaps too similar) to Neon Genesis Evangelion. In the background shadowy people have hatched a rather ornate plot. The tools are hyper technologies, way beyond anything our "science" would make possible, which actually trace their roots back to spiritual and mythological origins. And the battle is as much about the prophecy, with everyone trying to twist it to their own ends, even though no one we follow fully understands all that is going on. And, ultimately, the battle is mostly about the minds and souls of those on the front line. The ones who will be the point at which power is concentrated, and who have themselves been manipulated and twisted in the process.
What this means is that there is a lot more character drama than one might expect, and indeed a much larger cast. A great many of them are interestingly weird, others are obsessive and driven, and a goodly number are both. Of course at the same time there's no shortage of fatal flaws around. This is especially true for Ayato who (surprise, surprise) ends up being at the center of events. This is sometimes good, but I would have to say it can also wear over time. Just like Shinji in Evangelion his constant internal discord sometimes borders on the irritating, especially because it extends over so many episodes and tends to repeat itself... and besides, he's got a god-mecha, how unhappy can you be?
Of course part of the reason is that he can't actually become a "complete" character until the last episode where, just like in Evangelion, unbelievable things are expected to happen. In practice this generally means outrageous symbolism, massive death and destruction, and ridiculous depictions of excessive energy. I'm not going to say what does happen, but basically the show spends all of its time and effort building up to whatever it's actually going to be. As such the show is sort of stretched, we're always waiting to find out what the heck is being hinted at. This means it can be reasonably addictive, the mind desperately scrabbling to "crack" the symbolism and fill in the gaps, but it also means that the show lives or dies on the strength of the solution it can produce. And, in all honesty, I'm not sure it was actually worth it.
The core problem is that the solution being represented is just too "large" to actually make sense from a human perspective. Both sides of the conflict have rather more mundane solutions, even something as exotic as peaceful negotiations, that might provide more reliable outcomes. Meanwhile much of the actual story ends up having very little to do with the end result, it effectively could have happened round about episode 5 and still worked. Meanwhile the ultimate conclusion is distressingly domestic and merely makes concrete something we've been waiting to see for 20+ episodes, and which once again could easily have happened at around episode 5. Of course this assumes you can even accept that the ending makes sense, that so much power exists, which is far from trivial.
So in summary it is probably worth watching if you like a thriller type story. Watching various characters bounce off each other while little clues and insights emerge. Ultimately I don't think the clues actually end up being that relevant, but some of the more enthusiastic fans on the web-sites disagree and enjoy arguing the minutia. And some of the drama is quite intense, especially the episode with Hiroko, although I couldn't help feeling it partly ended that way because they'd written themselves into a corner. There's also a rather decent number of attractive females which doesn't hurt. The action is dull, the choreography on the conflicts is boring and the power levels off balance. Rahxephon is basically indestructible and the rest of mankind's military might is laughably inadequate. Some of the design work is interesting, but of course that further hampers any ability to animate it. And ultimately, as I've hinted, the build up is more interesting than the conclusion.
The production is rather disappointing. It does character artwork quite well, which suits the content, but much of the technical design is drab and lacking in detail. Some of the enemy mecha are extremely exotic (very much like the Evangelion angels), but not cool or particularly interesting in action. The more human technology is even less interesting, it just doesn't feel fully thought out or developed and the dingy palettes and unexciting atmospheric and motion effects make it feel lifeless. As mentioned the power balance is also way off, so there's little sense of danger, although there is massive destruction of unimportant extras. The voices are good, although to my mind the dialog didn't help establish character for a lot of them. The opening music is excellent, but the "singing combats" end up being just a single vocal note which seems sort of anti-climactic.
A "mystical mecha" movie in which a powerful machine, and it's emotionally confused pilot, are actually just pawns in a much a large scheme to change the world. It's a lot like Evangelion except it has more characters, less intensity, poorer action and an equally excessive conclusion. On the positive side there is more warmth to the characters, so some nice drama can be had. Don't bother watching it for mecha action though, as that is one of the more disappointing parts despite the generally decent production values.
I haven't looked around that much (it's late) but the only review from a regular source is one from Mike Toole at Anime Jump! (the review) but it's only on the first volume, and even then it only manages a moderate mark.