I used an acrylic yarn and it turned out fine. I did add one thing to the pattern however. I found the band around the bottom of the hat stretched out as it was worn so I stitched in a grosgrain ribbon to keep it sized.
The bill on this hat does not have any stiffening but it is a large bill that goes around the head. It reminds me of some of the hats worn in the colonial days and they also had soft bills. The fit is more like a bonnet rather than a baseball cap.