Here is a link to a 'flyer' put out by PROFECO, the Mexican agency that is supposed to look out for tourist, I think.
http://www.profeco.gob.mx/revista/publicaciones/otas_pub_06/timeshares_abr06.pdf
http://www.profeco.gob.mx/revista/publicaciones/otas_pub_06/timeshares_abr06.pdf