Rosebay willowherb is also known as fireweed due to its habit of colonising areas devastated by fire. It also flourished on bombsites during the Blitz. It's not usually a problematic weed. You can easily pull it out when it pops up. If you take over a neglected patch of land there might be plenty of willowherb growing on it, but it's nothing like as troublesome as bindweed or couch grass or any of those difficult weeds. Do pull it up before it sets seed though, as it makes downy seeds which float up and spread everywhere.
I think it's a very handsome plant, and apparently it was introduced as an ornamental. I'm not at all surprised, and I don't know why we don't still grow it as a flowering garden plant. I think it looks a lot better than some of the things you can buy in the garden centre. I've seen some references that say you can eat it, but it doesn't seem to be a great delicacy. You can eat anything that isn't actually poisonous, but whether it's worth the effort is a different question.