I am a Indian citizen, traveling to Mexico via Frankfurt (from Mumbai) with a multiple-entry Spanish Schengen visa. (The return journey would be Mexico-Munich-Mumbai.)
As per official Mexican Visa authorities (link doesn't work anymore, but here's an unofficial copy):
...Holders of any valid U.S.A., Canada, United Kingdom and Schengen Visas (any nationality) DO NOT require a visa to enter Mexico on tourist, business and transit purposes only....
And that was confirmed to me by the Mexican embassy also.
I have applied for and gotten the Schengen visa, for the first time.
But my query is:
I am not sure as I have never entered a Schengen country and the German authorities would consider my first point of entry as Germany and deny my entry as my Schengen visa was issued by Spanish authorities and on top of that I am travelling to Mexico!