Mexicana is Mexico's primary airline and provides a range of domestic flights as well as flights to numerous locations in North, South and Central America, with its two main hubs existing in Mexico City and Guadalajajara. The service is reasonably priced, and is often offered in conjunction with the Likes of American Airlines and British Airways to provide domestic and international connection flights from within Mexico as part of a transatlantic flight package.
I have used Mexicana to travel to a range of locations throughout central america, including Guatemala, Guadalajara and Mexico City, and have found the service to be efficient and reliable, having experienced no problems either with delays or misplaced baggage. The airline has a number of large Boeing 767 planes for its long haul flights, but I have only used the smaller and less luxurious Skybus planes for short to medium haul flights myself, and whilst these are not uncomfortable, neither are they spacious, and I have on occasion been unlucky enough to find myself in non-reclining seats, which can make things considerably more cramped. The staff are genearlly very courteous and efficient, and they generally speak a sufficient amount of English to cater for English speaking passengers, although they do of course predominantly speak in Spanish, as you would expect.
Whilst not the most comfortable of flights, Mexicana's short and medium range flights are reasonably comfortable all things considered, and passengers are well looked after with free meals, frequent snacks and free alcoholic beverages (unlike on bigger airlines such as AA, which charge you 6 or 7 dollars for a can of beer nowadays). Prices are very reasonable if you book your flights early enough, and all in all Mexicana airlines offer a decent level of service and seem to be pretty reliable in my experience.