Charles S.
Yelp
Via del Pigneto has become very trendy over the years, for better or for worse. When I talk to Romans about eating in Pigneto, they'll always mention Primo or Necci. But Quarantuno is better than both of those restaurants, hands down.
Their menu is creative, using ingredients that are not often found in traditional Roman cooking. For example, a pork loin with mustard sauce here, an entree seasoned with ginger there, etc. But they also excel at the traditional stuff. For example, their pasta carbonara is the best in Rome. (And I say this after quite an exhaustive search -- I'm an American, but I lived for five months in Pigneto and ate all over Pigneto and Rome.)
Also try their specials, which change frequently.
One thing I particularly enjoy is the service. The waiters speak English very well and are very helpful, knowledgable, and friendly. But, as it happens from time to time on Via del Pigneto, sometimes there is some unpleasantness. One night when my wife and I were dining at Quaruntuno, a drunk guy started begging for money from the folks dining outdoors. The waiters were on him immediately, and were very effective at "convincing" him to move along, without ruining anyone's dining experience.
The one drawback -- and why I give it four stars instead of five -- is that the price is a little high for the portion size.