Recent experience with airport bus
I just took the bus from GIG to the city (R$12 to Ipanema) on Monday morning and back to the airport on Thursday afternoon. I think posters here are vastly underestimating the time it will take.
In both cases, it took me about 2.5 hours. The entire port area is a massive construction zone, with streets closed off and traffic at a standstill. On the way down, it took probably an hour to go from the big bus station in the Zona Norte to Candelaria. On the way back, it was about the same time to get from Santos Dumont to Candelaria. Also, on Thursday afternoon the traffic on the highway to the airport was jam-packed from Sao Cristovao to almost the end of the journey.
If you take the bus, just be aware to leave yourself plenty of extra time to make sure you don't miss your flight!