Is this the one with the prickly leaves and rather pretty small mid blue flowers? It's a tough wild flower but it does tend to take over and the big black taproots go down to Australia and are a bugger to dig out. Good for making a bright red dye, though, apparently!
I'd think composting the leaves would be okay, although I'd get rid of the roots unless you want it everywhere! Comfrey makes, as you know, great green manure, but I'm not 100% certain about this 'cousin'.