IMPORTANT - IF YOU TAKE BUQUEBUS... Let's say you are leaving MVD for Buenos Aires, like me... Make a Reservation for your boat trip. Arrive plenty early, as you will be taking care of all your immigration paperwork from the MVD/Uruguay side. When you are in Argentina, you just get your luggage and go! It's that simple, but expect to take care of departure tax and other paper work for departure out of Buquebus Terminal.
Carrasco International Airport is the main gateway for flights to and from the country. Road and bus services across the borders with Brazil and Argentina are good. But unfortunately there are no rail services linking these countries. A ferry and hydrofoil service runs between Colonia and Buenos Aires.
The city centre is best explored on foot. Local transport is predominantly by bus and metered taxi.
I visited Montevideo from Buenos Aires. Buquebus was the best option a nice 3.00 hours cruise to Montevideo.
The ferry is a modern boat with a cafeteria and a very good duty free shop.