Although Italian recipes often list parmigiano reggiano as a substitute for pecorino romano (and sometimes vice versa), the flavours are very different, and using one instead of the other will change the outcome of a dish (although that doesn't necessarily mean it will taste bad). Both are salty - because salt contributes to the cheeses' 'keeping qualities' (along with ageing the cheese, salt draws out moisture). Pecorino romano has the typical sharp, tangy taste of cheese made from sheep's milk; par- mesan has a rounder, nuttier, milder taste, and is made from cow's milk.
Pecorino romano, like parmesan, is hard and quite crumbly, making it difficult to slice into thin, even pieces. Don't get pecorino romano mixed up with pecorino sardo, which is firm (but not hard) and eaten on its own, rather than used for grating.
I like to use pecorino romano in a stuffing for grilled zucchini. Mix fresh breadcrumbs with minced garlic, Italian parsley, finely grated lemon zest, freshly grated pecorino romano and black pepper (no salt, because the cheese is salty). Cut medium-sized zucchini in half lengthwise and scoop out the seeds. Spread the breadcrumb mixture over the zucchini, mounding it slightly over the squash, then drizzle very lightly with olive oil. Bake at 200 degrees Celsius for five to 10 minutes, or until the squash starts to soften, then place under the grill to lightly brown the topping. This breadcrumb mixture is also delicious when stuffed into firm, not-too-juicy tomatoes (the Roma variety is ideal).