Why? Mexico is a fairly large country with a wide variety of terrains and many overlapping histories to explore. Beach towns line the Pacific and Yucatan coast and have remained fairly safe amidst drug violence issues in much of the rest of the country. Mayan, Toltec and Aztec ruins can be viewed throughout the country as can building from the Spanish colonial era. Costs are low and tourists are very welcome, but unfortunately drug violence has affected tourists in almost every state at some point.
Where? Because of changing safety issues related to gangs, drugs, and illegal immigration, I would check the website of the US State Department before planning any trip to Mexico. Baja Sur is one of the few areas that seems to always remain free of the major problems and is a good beach choice. The beaches along the Gulf of Mexico are calmer and well known for warm, turquoise waters. The central area of the country includes many scenic colonial towns along with the capital, Mexico City. Volc anoes, canyons and other natural parks can be found scattered around the country.
When? December to April holds the best weather and the major Catholic holidays along with their elaborate festivities. May through August the country gets hot. July and August see occasional monsoons along the Pacific, while September and adjacent months are peak hurricane and rain season for the Gulf coast.
How much? Mexico has been a good travel value for many years. Backpackers can get by on around $40 USD a day. Prices jump up to around $90 USD per day for nicer mid-range options, but you can find some neat colonial and boutique options in this range. Luxury lifestyles can find hotels and fine dining for about $150 USD a day.
How long? If you wanted to see all of Mexico it would take quite weeks or even months. Many people spend just a long weekend enjoying the resorts and beaches along the coast. I was able to make a loop around the highlights of the Yucatan peninsula in just over a week, which was a nice introduction to beaches, ruins and colonial towns. But I still haven’t made it to Mexico City despite half a dozen visits to Mexico. The more time you have, the more areas you’ll be able to explore, but since the country is so big you’ll have to prioritize and think about flying from one region to another.