We are told that the Journey should not be interpreted literally: "where you are going is a state of mind: what matters is how you travel," says Buddha. To acquire the scriptures, the pilgrims must achieve Enlightenment (the inspirational value of Tripitaka's sainthood far outweighs the material value of the scrolls?). However, just as the monsters that oppose them are demons, allegories of evils such as greed and selfishness, so the pilgrims themselves are not complete individuals but different facets of the human psyche. The troupe's journey to fetch the scriptures is thus a human life and its journey towards Enlightenment.
"There are no extremes which are right. There is only one way, and it is in the middle" the Narrator tells us, several times, stressing the need for balance. To achieve Enlightenment each disciple must balance his failings with his strengths, and only when each member has managed this will the group reach its destination.
So the good news: Tripitaka is kind and pious. Bad news: he's also foolishly naive and over-optimistic.
Monkey is bold, confident and cunning; but prone to mischief and violence (often at the same time!).
Pigsy is obviously a lecherous glutton, but he's also physically strong and hard-working (though explicit in the text, this is less obvious in the TV series - but watch who always carries the backpack...)
The Dragon-Horse I'm not sure about yet; basically he's a big coward, but very empathetic and a mediator in conflicts.
So far, so obvious. What puzzled me is that the role of Sandy seemed to be shrouded in mystery, for no real reason. In the introduction to his translation, Arthur Waley complains that Sandy "remains singularly colourless and ill-defined throughout" (IIRC), and a number of other texts on the Web (even specifically TV-series related) echo that stance. Confoundingly, the "Journey to the West" was already an established folk-tale before the novel (like Robin Hood, or King Arthur); I'm in no position to speculate on what the author might not have bothered to spell out as he could have expected his audience to already know it. I also don't know which aspects the TV scriptwriters added themselves.
Sandy's positive traits are easily identified: he is educated (he can read and write, and can do arithmetic) and aspires to scholarship. He doesn't covet material wealth, preferring contemplation and philosophy ("Even in heaven, where will Sandy find somewhere to philosophise? His needs are modest: peace and quiet, and an ordinary life."). Although its not clear how advanced his knowledge really is, Sandy is aware of his limitations. Unfortunately, he is also the cliché "dry" intellectual; aloof, almost autistic, in failing to engage with people and often unable to see the human problems (e.g. he can see that the maths problems are wrong and can correct them, but doesn't ask why incorrect sums have been hung up around the village). Monkey regularly complains about Sandy's "philosophizing" but when Sandy does come up with a plan, it is usual horribly impractical (e.g. the Tripitaka's-dream-demon-extraction cenrifuge...). Luckily (for everyone else) Sandy very much the theorist and only rarely takes action proactively: "Sandy, using all the powers of his intellect, guessed something was wrong. He acted decisively and at once: he told Monkey to take care of it."