I wanted to build one so badly - I remember those ads. One day I
  saw the extension cord though then I was like, OH COME ON! I
  credit/blame David Horowitz. He had a show called "Fight Back!"
  which was a pro-consumer show which exposed common scams and
  gave helpful advice. I loved the show. He never mentioned the
  hovercraft... I just noticed the extension cord in one of the
  ads once and it 'clicked in'.   I still want one though. You can
  make one with leafblowers - saw it on Mythbusters. Noisy sucker
  though smile emoticon