Chipmunks are generally non harmful to humans and the environment. They provide an important food source for not only domesticated cats, but for bobcats, coyotes, hawks, weasels, raccoons, minks, large snakes and even red squirrels. Chipmunks may also play an important role in dispersing seeds from plants that they are eating and storing during their travels, and even aerating the soil as they dig!
Their digging, however, can cause problems for gardeners. In particular their affinity for digging along sidewalks and foundations can affect the stability of these structures.
All in all, the benefits far outweigh the detriments of the Chipmunk.