It's pretty good. A bit 'basic looking' but efficient, dominated by orange trains & grey tunnels, very little advertising and dead cheap - 5 pesos (30p) for any distance. There are stories that it's dangerous to ride at night, but I've found it fine. No more dangerous than the NY subway probably. You just have to be sensible.
In addition to the name, each station has an icon, representing an important landmark or activity associated with the neighbourhood. They - and the M logo - were created by the American designer Lance Wyman, mainly to help illiterate passengers. He also, incidentally, designed the 1968 Mexico Olympics logo.