IME Brazil has a bigger spread between buy and sell rates than one typically sees with other currencies against the USA dollar. Also, when I tried to convert Brazilian money to Argentinian cash in Buenos Aires, I didn't seem to get a very good rate either (in an official exchange place, although I checked rates at several of them).

I agree with the advice to avoid the airport ATMs. Change USA dollars in cash if needed. Also, in Rio, be careful about people hanging around near ATMs and the doors of banks. Many hotels seem to have ATMs in their lobbies or business centers, but I didn't investigate fees.