Pimentón is an essential ingredient in Spanish cooking. So many of the recipes I post here will call for it that I feel I should say something about it first. There is really no substitute for it. Regular paprika won't do. If your local supermarkets don't carry it (mine never do), you can order it on line. We stock up on pimentón whenever we're in Spain.
Pimentón comes in three varieties--sweet (dulce), bitter sweet (agridulce), and hot (picante). Recipes in Spanish cookbooks often specify which to use, but I almost always use the picante, because we like the heat, which, by most hot-pepper standards, is really still quite mild. It is certainly less piquant than most chilpotles, for example. However, some brands of pimentón--and, in my experience, even some individual cans of pimentón--can be spicier than others. So you may want to begin with a little and work your way up to the amount of pimentón you like.
Spanish cookbooks (and Spanish cooks) often suggest buying only pimentón labeled de la Vera, meaning from La Vera, in Extremadura. In my experience, this is good advice. Pimentón not from La Vera can be pretty good, but pimentón de la vera is always better.
For vegetarians, vegans, and anyone interested in eating less meat, pimentón is an especially valuable spice. When combined with olive oil and garlic, it adds a depth of flavor that meatless dishes often lack. When a recipe calls for a small amount of chorizo, I will use extra pimentón, and use slightly more olive oil and garlic. Pimentón and fat, after all, are what give chorizo much of its flavor. The dish will certainly not be the same as it would have been with chorizo, but it is quite good, and sometimes better, as with lentil soup, the subject of a post in the near future.
One warning about using pimentón: it burns extremely easily. If a recipe tells you to add pimentón to a saute of onion and garlic, as a base of a soup or sauce (and many Spanish recipes will), it is best to turn down the heat to low, or even take the pan off the burner altogether, and let the oil cool some. Otherwise the pimentón will turn black and bitter.