I hadn't heard of them until I was invited a few weeks ago to one that happened last weekend. I was happy to be invited to a party with people I liked. Most of us went out and bought cheap ugly sweaters from Goodwill, Tuesday Morning, etc. (I would've tried to find one I already had, but I moved two years ago and didn't bring my ugly Christmas sweater(s) with me...) I had a good time. It was also fun kind of competing for the ugliest sweater, and it sort of relieved the usual pressure (even if it's only slight) to try to dress well, since we were all trying to dress poorly. I wouldn't say it's an essential part of the season, but I had fun, and again, I was just happy to be invited to a party.