At the recommendation of another review, which I now can't find, I used the services of Sao Paulo Airport Transfers on my recent trip to Sao Paulo. As a regular business traveller I really cannot recommend them enough, from the quality of car used, to the standard and courtesy of the driver.
My flight to Guarulhos was delayed by 2 hours and I was unable to contact anyone to let them know, but the driver was waiting patiently for me when I arrived, and assisted me with my luggage.
The most outstanding aspect of the service was when I was taken back to the airport. When leaving the vehicle my Blackberry must have fallen out of my pocket, the driver spotted this after I was inside the terminal and actually came looking for me at the check in desk. Fantastic!
The website of the company is reasonably easy to remember, it's www.saopauloairporttransfers.com - they provide taxi transfers from Sao Paulo Guarulhos airport(if you hadn't worked that out already!), they may also offer the same service from Congonhas, but would be best to check that with them.