We went to Las Montanas recently for a family birthday lunch. The service was a little spotty, with orders being taken and not followed up on. There was also some difficulty with getting the hand made tortillas and salad dressings we wanted.
Even so, all our food eventually got to the table, and we were happy. Upon reflection, I would say that the most enjoyed items were the traditional Mexican ones: carne asada, fajitas, enchiladas, etc. Less enjoyed were the Americanized items. So I would recommend staying with the traditional Mexican menu choices.
Las Montanas does feature mariachi bands and other musical events, so you might want to check ahead of time, according to your preference.