I can't think of any subject I'm interested in with a worse signal/noise ratio than druids. From growing up in the UK I have an iconic image of a priestly class in white robes brandishing a ceremonial sickle beneath a sycamore tree (or an oak or a rowan). That's mostly from horrible histories and primary school lessons on roman Britain.

As an adult I read that it is essentially impossible to know how much of our written mentions of druids are Roman libel (especially re: human sacrifice) and there is the added interference of pagan revivalist mythmaking. At the risk of asking too broad a question, what do historians currently think about druids (even if they disagree with each other)?