Serious Eaters much more knowledgable than I am about pizza have already expounded upon Rubirosa's pizza greatness; check out Adam Kuban's full review and Kenji Lopez-Alt's Slice review. I'm basically here to agree with them.
If you're looking for a chill restaurant in Nolita for a group lunch, Rubirosa is a great choice. My party of five had no problem getting a table at 1 p.m. on a Wednesday afternoon (they can seat larger parties in their back room). Their made-for-sharing pizzas come in large (eight slices) and small (six slices). We ordered two large pies and still had a few slices left over when we were done, despite the pizza's seemingly not-that-filling thin crust.
The classic (small, $15 / large, $21) features a light layer of tangy tomato sauce balanced out by sparse blops of mozzarella on a thin, crisp, and pliant crust.
For something a bit meatier, try the mushrooom (small, $17 / large, $24), a white pie topped with slices of mushroom, strips of prosciutto, diced pea pods, and pecorino.
The simple, well balanced mixed greens salad ($9) of arugula, thinly sliced cucumber and radish, halved grape tomatoes, and pecorino shavings in an oregano vinaigrette makes a nice light side to the pizza.