Visa was not required for our short vacation in Isla Mujeres, Mexico (visa not required for stays shorter than 180 days for US citizens, please refer to the US Passports and International Travel website for more info). We flew into Cancun International Airport and went through immigration and customs process there, as there is no airport on the island. We had to fill out 2 forms: one form is per individual, which is the document needed to enter Mexico, and the second form is per family, which is for customs.
There were 3 lines for immigration, which were all long. We went from line to line to find the quickest line, and I would say that the average wait time was 40-60 minutes. The whole process was very simple and fast when we actually got up to the window. The agent was efficient and friendly. We had to go through customs, which involved having all our bags and luggage go through an x-ray machine again, and handing the customs form to the agent. This was also a quick process. We did not see an ATM for withdrawing pesos (only saw one for US dollars) on our way out of the airport. There were many sales people and vendors along the way out of the airport, and once we were outside, we were met by another wave of cab drivers, other transport drivers, random people, etc., as well as the heat and humidity.