They rate at 2,000 to 5,000 on the Scoville Scale, which puts them, according to that measure, at 2 peppers out of 5, which, as stated above, is a solid medium in heat. Cooking with the seeds increases the spiciness, and every once and a while, like with all chiles, you'll happen on a mouth searing pod. The spiciness from the traditional capsaicin can be pretty easily detected by smell...so sniff your peppers! This pepper hails from the valley of Mexico in the central part of the country and is very popular there in both chiles pastes and salsas. They often make up a simple and thin sauce for tamales in that region. It's closest relative is the much rarer Mexico City chile the Pulla, which is much hotter! Both are members of the Caspicum annum.
Famed Chef Mark Miller penned a helpful book of chiles entitled The Great Chile Book for Ten Speed Press, in his describes the Guajillo in the following manner:
Thin fleshed; green tea and stemmy flavor, with berry tones. A little piney and tannic, with a sweet heat. Commonly used in salsas, chile sauce, soups and stews.
I'm really not at all sure about the "stemmy flavor," I would agree, or say, that they have a very earthy taste and the heat definitely builds toward the back of the throat. In addition to the above mentioned uses, they are excellent in traditional Mexican dried chile relishes, that these are all the rage in my house. Additionally, when pureed that can go into cream sauces, salad dressings, marinades and dips; the make a really interesting sour cream to go over tacos and, when proceeds to a powder while dry, are an excellent traditional ingredient to native hot chocolate.
Nutritionally, they are quite high in vitamin A, Phosphors, Vitamin K, and some B-vitamins, especially Niacin and Magnesium and has some easily soluble iron. It is actually used as a digestive aid. Each pod ranges from about 15 to 20 calories with seeds (which have most of the oil in them). Each also has about 8 grams of carbs.