I believe her technique is effective because it tries to combat hoarding using the same kind of emotional response that gets us to stockpile on stuff in the first place.
Others will suggest attacking the problem with logic: “Do you need this now? No? Get rid of it.” But stuffocation is as much a problem with our emotional attachment to things as it is to our evolutionary hardwiring. In my case I was finding it easier to find reasons not to keep something by doing what Mari suggests: take items into your hands and ask yourself if it actually brings you joy.
Came to post this, actually. It's an incredible technique. She recommends doing it all at once, though we've had to split up the work over several weekends. I've tried in the past to reduce clutter, but her method is the first I've tried that works.
We donated a ton of clothes, and the best part was that afterwards I had a new appreciation for the clothes I kept -- like, "oh, I've got some really nice stuff and I hadn't realized it for a while, because there was so much clutter."
It might be largely due to the fact that her technique involves actually holding every item you own before you decide whether or not to keep it. And you end up keeping the stuff that matters to you _now_.
Never did I think I would be interesting in house tidying, but her techniques are effective.