Los Algodones, Mexico
Hi Stephen. We went down a couple years ago from Canada specifically for dental. A lot of Canadians and Americans do this. You will find the better/most popular dentists are booking a week or more out. So yes you'll want to do your research and get recommendations, then book ASAP.
Crossing the border is quite easy. What you'll probably want to do is pay for parking on the American side and walk across. As long as you are going back before too late (later afternoon into dinner time seems to be the busiest time to cross back) it doesn't take too long.
For example, we had to go down 2 or 3 times over the course of about 10 days. Our first appointments were earlier - 9 AM so going to and coming back was quick. We had a mid afternoon appointment and we stayed for a late lunch after that and walked back around 4:30. Here we waited about 45 minutes in line before crossing customs.
Final appointments were around 11 AM - again no problems crossing in and back.