Mexico City has a great subway network, with 175 stations connected over 11 lines. Even if it has 3.8 million of users every day, it has been completely surpassed by the growth of the city, as you can see in this picture from Periférico, the main ring road.
The problem is compounded by two factors: sprawl and quality. The subway only covers the Federal District (and not completely), while the metropolitan area includes also municipalities in the states of Mexico and Hidalgo. Furthermore, most people who can afford a car try to avoid public transport, as it is perceived to be unreliable, dirty and unsafe (as anybody who has used a microbús can attest).
I would like the European model of reliable public transportation for the masses to be available in Mexico, rather than the American automobile-intensive way, but it needs more political will than there currently is.