...... but useful weed - Think of a sand dune or a bare, scraped clean wound in the earth. Quackgrass is like a scab growing over the wound, something quick to weave green leaves and allow the soil below to heal. And it has been used as both food and medicine. But in a garden it's a disaster. One quarter inch bit of root and it's off again, spreading through and around every plant in its way.
Not the worst weed. That title is reserved for horsetail and sheep sorrel, in my opinion, though sorrel seems to be suppressed by a thick growth of other plants. And at least the root is white and fairly thick, easy to see and tough enough to pull up in clumps.
Fortunately, not too hard to remove IF you remove everything else. In clear ground, you can dig out as much as possible. The problem is that a quarter inch left in the soil will renew the whole clump. The remedy? Go back a few weeks later and pull up any bits that sprout. They'll be easy to remove, no roots yet.
Unfortunately, most people look at the sprouts and think "Oh, no. I'm not going through all that work again! It's just a few sprouts, I'll get them later." First, just pulling those sprouts takes almost no time. Second, once you let them get rooted you're faced with the same problem, leftover bits left that poke up again.
If the quackgrass is enmeshed with shrub or perennial roots, there isn't much you can do but dig out the host plant, throw it out or go through a lengthy process of picking out the grass. Often not worth the trouble.