What's produced at hackathons?

  I've never participated in a hackathon, so I can't tell whether they produce anything or not. I'm assuming that at least some of the readers of this blog have been to one or more.
  What's it like?
  What kind of software is produced?
  Is any of it useful?
  Were any commercial products hatched at hackathons? Were any of them successful?
  My intuition says that they're pointless exercises. I have at times gone on retreats with the idea of emerging with something useful or marketable, but I have to say the big leaps in software that I've seen or taken part of, come with steady daily work, when you've built up a head of steam over weeks or months.
  It seems to me that the hackathon idea is more a dream of investors or marketers, that they can get a bunch of programmers in a room to invent something they can make money with. But that's just my impression.
  If you have a story of a hackathon, please tell it.