Julius Caesar bigged them up in his self aggrandising 'The Conquest of Gaul'. As he did the by then old fashioned war chariots used by the Ancient British. As has been said earlier in the thread Suetonius made mention of them in the later Claudian invasion of Britain. How important they were we'll probably never know but they certainly existed, however I suspect that modern 'Druids' have little more than the name in common with those of 2000 years ago.