Just returned from holiday in Positano, to share a few experiences:
Donna Rosa up in Montepertuso. Caters to Italians and foreign tourists alike. Basically rather simple italian food excellently prepared in an upscale but still cosy ambiente and served by very friendly staff. A little more pricy than down in Postano (and you have to add the cost for transportation) but absolutely worth the visit.
Don Alfonso 1890 between Sorrento and Positano. Very famous. very upscale, expensive (there are very adequately priced wines though but transportation again adds a lot to the over-all bill) but to us a little bit of a disappointment. We considered the food not that exciting or innovative (albeit of course very good) compared to other similar places in London, Spain, France or even our native Austria. Thus we had rather missed Don Alfonso than Donna Rosa.
Il Capitano on Via Pasitea 119, near the western end of Positano. That one we considered the nicest "normal" restaurant in town. It is a little bit hidden (do not enter the terrace of the Pizzeria to its left) - you have to enter through the unobstrusive entrance proper to arrive in the dining room or, even better, the lovely terrace with its view onto the sea. The menu offered a slightly more refined fare than most of the other streetside restaurants and everything was simply delicious. We did not like the wine too much but since we chose something unknown to us without consulting the waiter it was our own fault mostly. The prices weren reasonable, the staff not overly friendly but OK, there was general purpose (and thank god, not schmaltzy italian) music and there is no traffic noise, also not easily found on a restaurant along the main street of Poistano.
A last tip if you happen to be in Ravenna: go to the restaurant on Piazza Luigi Einaudi (right behind Piazza del Popolo; we forgot the name, but the one down on the ground NOT the upscale one on the second floor of the palazzo facing it). It looked like an average place in a touristy area (though we did not find too many restaurants in all of Ravenna) but offered delicious food (from rabbit to seafood, like salad of marinated polpo) at very reasonable prices.