While there are many places that accept credit cards here in Montevideo, you will often see signs like this one: 

Visa  pretty much has the market  cornered.  All of the locations that take credit cards accept Visa.  Probably about half of those also accept Mastercard.  I was at the hardware store yesterday buying a few things and as I pulled out my Mastercard, I got the response of  ”No.  Solo Visa, por favor” (Only Visa, please).  So I paid cash.  

Before you just grab any Visa from your wallet, make sure you know what your bank charges for foreign transaction fees. Some banks are as high as 3%, others are 1% or even 0%.  Make your money stretch a bit further.