hackathons seem to divide into two main types - one where people get together to build stuff and advance the state of a (usually popular) open source project via a concentrated sprint, and the other where people or teams compete for prizes.

personally, the latter strikes me as betwixt and between - if i wanted the competition, i'd rather enter a pure programming contest with explicitly artificial problems and well-defined judging criteria, and if i wanted to build something i'd rather be doing it cooperatively than competitively.