We stopped in Piazza Navona upon arrival in Rome on the day after Christmas. The market was bustling with tourists, filled with vendor stalls, a carousel, and festive balloons. It was a great way to spend a little time while waiting to check into our hotel. As others have mentioned, it is touristy and a little junky, but we still had a great time experiencing the giant Nutella donuts and checking out the Befana ornaments and nativity scenes. Chestnuts were roasting as well.
I'm not sure what the piazza is like without the Christmas market, as the appeal for us was primarily the holiday festivities. I'm rating a 4 for the Christmas market, but probably less on other days.