A friend from Puerto Rico could not understand Mexican spanish...they spoke too fast with too much slang.
After studying Spanish on my own for a number of years and four semesters in college, I am able to understand a fair amount of Spanish spoken on documentaries. The Spanish speakers on the docs tended to speak at a reasonable pace and enunciate well.
However when trying to understand Mexican soap operas, I was/am almost completely lost. Los actores hablan demasiado rapido.