Over the last 12 years, I've played about 175 RPGA games (and run about another 160 or so). In general, I find that out of six games, I'll have one bad judge, one incredible one, and four good-to-decent ones. I live for that incredible DM, and try to remember who they are so I can keep in touch with them. I've made a lot of good friends that way.
We have a theory that says that a really good game has three components: the judge, the module, and the tables. It requires two out of three of these things to make the game good (a good judge and players beats a sucky module, for instance), but that the judge trumps. A good judge can enliven both tired players and a poor module, and a bad judge running a great module can make things tough even for fantastic players.
That's why I tend to run games at cons; it's all selfishness. When I run a game, I know that at the very least I'll amuse myself (and hopefully the players!), thus guaranteeing my own fun. Because of this I seldom have a "bad" con; I just enjoy the challenge of running an unfamiliar adventure for strangers, and I'm seldom disappointed. Even better, I blatantly yoink DMing and playing tricks from my players that I think are cool. The RPGA (hi, Adso!) is solely responsible for any DMing ability that I might have.