Good idea overall, reminds me that most material possessions are unimportant. I've done a pretty good job at not buying things since the start of this year, have not been to Domain for clothes shopping (or retail therapy) for even once. Pretty happy in my jeans and old sweaters everyday, feeling less self conscious. When summer comes around I'll probably dig out a bunch of dresses that I only wore a few times but never want to wear again due to size / fit / style. That will be the time to head to goodwill.
I do think the author's perspective gets more and more extreme as the book goes on, and that's why I stopped 2/3 into the book. It's a bad idea to throw away something you think you 'might need', only to find out later you actually need it and go to store to buy it at full price. That way you will pay for it twice. If you think you might need it and it's not an eye sore, just keep it. Throw away the things you regret buying or are sure won't use again.