Some great talks, interesting presentations and new ideas. Also, you get to meet very interesting people and get to pick their brains or just hang out and enjoy their strange and wonderful personalities.
However, the best is when you discover people who are true hackers. By that I mean people with a certain state of mind who take a creative approach to solve problems.
Here's an example which proves that a real hacker does not need a computer but only his brain:
And yes, this guy was hacking away at deciphering some message.