In 2007, the Mother Earth News tested eggs from 14 flocks of free range birds, and compared the results to the USDA published nutrition information for commercial eggs. The results were striking (pdf).
Read the full article (with big pop-up ad).
Right now, we are putting up a pasture fence to keep the birds safe from predators when we aren't around (and to keep them out of the garden!) Until then, the girls have been out when we are home. It's amazing how active and curious they are, and after they've been out browsing for a day, the flock eats about half the pellets they would if they had to stay inside. Foraging on bugs and plants (right now including groundfall fruit and anything left in the garden) is a big part of their diet.
What the chickens eat really does make a difference in the nutrition and flavor of the eggs. We don't feed them commercial feeds with animal byproducts.