Some wild mushrooms are edible, and some are choice edibles that have been enjoyed for thousands of years. Others may have strong medicinal properties. However, there is one inviolate rule about gathering and consuming wild mushrooms: "when in doubt, throw it out".
However, once you learn to identify and cook wild or exotic cultivated mushrooms, you will find that they impart an earthy, sometimes meaty, flavor to foods known as umami. They can also be high in protein, B and D vitamins, rich in anti-oxidants, and are fat and cholesterol free. Some mushrooms have proven to be anti-viral, anti-fungal, help control cholesterol, boost the immune system, or fight cancer.
For further information, please visit the Puget Sound Mycological Society's page on edibility, or the Wikipedia article on medicinal mushrooms. You may also review the list of mushroom species included in this site.