The cambios at the Playa Dorada Plaza will cash Amex travellers cheques.
There is no fee but the exchange rate is lower than their cash rate by around 1 peso and their cash rate is lower than that of a good cambio or bank by around 1.5 pesos so people who change their TC there lose around 6-7%.
If you don't mind lower exchange rates and wasting time and you want piece of mind, TC are fine.
I'll stick with US$ and ATM.
Most ATM fees are not hefty unless you are an idiot taking out 1000 pesos or some other piddly amount per transaction.