The plot works on many levels; it's straightforward enough for the general kid to understand and enjoy, but is piled under levels of wit and more jokes that rely on understanding more of it, making it balanced and still funny the 150+th time I watch it.
The songs are catchy (as can be well-expected from the good man Elton John), the characters are lovable yet total sleazeball con-men, and the humour is on many levels; and with it, it brings many good, memorable lines ("'For three days?!' 'YES! Don't even breathe!'" and "'You're buying your own con!' 'At least I'm not DATING mine!' '... oooh, low blow.'" come to mind).
As a basic rundown, there's Miguel, the fun-loving, more light-hearted of the two con-men; he tends to appreciate the beauty in fun and people. Tulio, the other half of the duo, has a bit more preoccupation with material possessions and wealth, though he still remains human. Then there's Chel; the seducer from the city of gold, able to help the two (at a price). And our main antagonist? Tzekel Khan (spelling unsure), a rather nutsy high priest and speaker for the gods, who proves to be... well, a basket case.
The animation pulls itself off well; the movie is bright and colourful, but not a kiddie flick at all- rather, it's humorous on all scales- my friend's 6-year-old-brother, I, and my 55-year-old dad agree. The Road to El Dorado is enjoyable on all accounts. If nothing else, consider renting it.