It's worth trying to get into On Sampu -- it's a special neighbourhood place. We liked it so much I suggested they open a pizzaria in Holland but they told me that they are already busy enough in Cagliari. We ordered our pizza and sat down to wait and wached as hundreds of take-away pizza's were carried out by delivery boys. Still, even without a reservation, we got our pizza in about 40 minutes on a busy Friday night (it seems everyone in Sardinia eats pizza on Friday night?) without a reservation. In October, they were featuring pizza with delicious wild porcini mushrooms.