I attended my first Hackathon last year (in fact, the first Hackathon I attended was one that I hosted!). It was a really incredible experience. So many more people turned out than I expected, and the quality of the work was really impressive. Most people who showed up were University kids, and they really killed it. In the course of 5 or 6 hours we created 15 or more three.js based browser games (there were close to 50 people there).

My only regret has been that I've been too darn busy to host another!