I don't mind hacking something together and not getting paid, but what bothers me is that hackathons are very rarely about what actually gets made - now that many have thousands of dollars worth of prizes, it actually behooves hackers to put a lot of effort into presentations rather than development, and if you don't win a prize you feel like you've wasted your weekend.

I've found civic hackathons like Random Hacks of Kindness (http://www.rhokaustralia.org/) to be a lot more fun though.