Excellent on the compost heap also, turns heap into brill compost. Also can use to make liquid fertiliser, the same way as you would with nettles.
Father used to chew the leaves, I DONT recommend that though, he thought it was good for him, (its also known as knitbone), I've since found out it contains a carcinagen and, guess what he died from......
Comfrey can also be made into a poultice and is very good for as an aid in speeding the recovery of ankle sprains and bruising. It was also know as 'knit bone' as it is also supposed to help in the healing of minor bone fractures.
It's all to do with the alkaloid content of the leaves.
Its described as a miner plant, since it puts down long roots and "mines" nutrients. Don't plant any where you don't want it permanently it's difficult to remove.