I think it is a nice place to spend your early Saturday morning. It has a lot of movement and it is a frequented place of people that lives in the city. Very interesting.
I'd put this at the top of the list if you are in Campinas. Good place to buy unique, interesting gifts for others, as well as sample some of the excellent Brazilian street food. Plus it's nice to be able to stroll outdoors while shopping without being in a mall, unfortunately this is a rare opportunity in Campinas.
Great market, feels safe. There is a regular police presence there during the Feira. Lots of different crafty items for reasonable prices. Lots of homemade goods and plenty to eat. Also many places to eat and little shops surrounding it.