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.