I know some number of people that have gone across the border into Mexico to have extensive dental work done for less than their insurance contribution would have been in the US. According to someone that I know well in San Diego who has had work done in Mexico, the dentist he uses is very professional — a dental assistant meets him in San Diego, travels with him on the trolley to the border, walks across the border with him, has a car to drive him to the office, and repeats the process for the trip back.
TP Wire Service reports that a company called Dentimundo is providing a bit more structure and formality to the process.
Certainly worth looking at whether or not you have dental insurance if you need extensive work done (maybe even worth looking at for routine items if you don’t have insurance). As with all things like this, exercise the appropriate level of caution.