Just spent 10 days in Cagliari for work - most of the food was catered, so I didn't have a chance to eat out much but there were a couple of places worth mentioning.
Had 2 really excellent meals at Trattoria del Porto which is located right in the port alongside the docks. Here they only do seafood but very very well - and the place was packed the two nights we ate there. No menus offered, the waiter just rattled off the evening's specials. We had a trio of starters - octopus salad with potatoes, olive oil, lemon, garlic and parsley - the octopus was melt in your mouth tender!!!, also had moscardini - which are like little baby octopus in a spicy tomato sauce - yum!, and tuna with sliced onion and olive oil. For first course, we had tagliatelle with vongole, bottarga, and fiore di zucca (that's fresh home-made pasta with clams, dried tuna roe, and squash flowers). This was soooo good - i love bottarga but had never heard of combining it with squash flowers - it worked really well - and to have it atop fresh-made pasta was just heaven! For the second course we shared a platter of mixed fried seafood platter - i know, it's a rich followup to the pasta, but it's a weakness - we can't turn down a good fritto misto when in Italy. And to finish it all off, a glass of lemon sorbetto. On the second visit, we skipped the antipasti (they were really good, but we didn't want to stuff ourselves) and went straight for the pasta - I had tagliatelle with lobster this time, which was delicious. Then for the second course we had grilled fish - which was perfectly cooked whole - and so fresh it didn't need any lemon juice or anything else for that matter - it was just right in its simplicity.
We had a great Naples-style thick crust pizza at Rossopomodoro on via Sasseri just up from the train station. I had found out about it by doing some research on-line, and was pleased to know they shared the slow-food philosophy. I ordered the gustosella pizza which was divided into four parts- fresh buffalo mozzarella and rucola; fresh cherry tomato and buffalo mozzarella; fresh pesto; and mushroom, proscuitto cotto, tomato, and mozzarella. Wow! It was really one of the best pizzas I've had in Italy - almost as good as the one I had in Naples. The dough had good flavour - almost sweet and you could taste the wood-smoke in the charred bits, the cheese was super fresh, and the tomatoes were very sweet. My husband went for the fried pizza - which was like a big fried calzone - he loved it, but it's too rich for me.
We also had pizza at the Oca Bianca - which came highly recommended by the locals - but was just no comparison to Rossopomodoro. The pizzas were huge, but with very thin crust and the dough was not very nice nor wood fired. Would not recommend it.
My husband also really enjoyed the food at L'Osteria on via Azuni in front of Chiesa St. Anna, which serves traditional sardinian food - but a little more upscale.