Mexico City has an excellent metro system.
It’s very cheap (possibly the cheapest in the world), clean, efficient and much safer than many other subway systems I’ve traveled on.
With the exception of peak hour, the trains aren’t too crowded and they run very frequently. The trains are similar to the trains used in Montreal in Canada in that they have rubber wheels, which makes the ride much quieter and smoother than normal trains.
Another way that the Mexico City Metro is similar to the Montreal Metro is that many of the subway stations have art displayed in them.
I’ve read that pick pockets are problem on the metro but I think that if you avoid the rush hour and keep your wits about you, you won’t have any troubles. The metro also serves as a bit of a marketplace as vendors (quite often blind) travel up and down the carriages hawking their wares. I didn’t see anyone buy anything from the hawkers on the trains, the whole time I was in Mexico City. Some people have a hard time making a living.