The average Internet service that Telnor in the northern part of Mexico or Telmex in the rest of Mexico sells, is good. You will get 5 mbps of speed. Obviously, you get much better speeds in the US, but here in Mexico, unless you are doing something on the computer frequently such as watching movies or downloading music, you are not going to be able to really feel the difference.
I can have a Skype call with someone in the US while I am here in Mexico and there is no delay whatsoever. The connection is perfect. In the major expat areas such as around Lake Chapala, Puerto Vallarta, Merida, Cancun, etc., the Internet connection is close to as good as it is in the US. My brother went to study in France and I heard that the Internet connection in Europe is much worse than in the States or Mexico.
Equally as in the States, if you go to any restaurant or café here in Mexico, most likely they are going to have Wi-Fi, especially when you go to expat areas. Every single restaurant has free Wi-Fi for them because that is how they communicate. It is very common that at the corner of some street there is going to be a Wi-Fi connection that you can use for free and the connection here in Mexico is very reliable. I would say the Internet connection here only goes out once in every 3 months.
(House facing the Pacific ocean, Ensenada, Mexico, pictured.)