My (oldish) herb book says "Many people chew a leaf on a hot summer's day, but others are allergic to it and in large quantities it is dangerous." It's not nearly as commonly used in cooking as it used to be in medieval and ancient times, but some people do use young shoots in salads.
Ordinary rue is a flavouring for grappa, but the little leaves in the bottles of "Grappa con ruta" are of Ruta chalepensis instead.