I qualify for Mensa, but have never joined, because "I, too, am good at taking standardized tests!" doesn't sound like a fun excuse for a party.

I try not to get overly snotty about Mensa, though. From what I understand (and from what people are saying above), there aren't really a lot of doctors, lawyers, engineers, professors, etc., in Mensa. I think it's because they don't NEED to belong to a "smart people club" -- they're in professions that give them intellectual validation and stimulation. The bulk of the Mensa membership seems to be made up of people who don't get that in their jobs, and so need some other kind of external reinforcement that they're smart.

Also, people who form a group based on being smart, but then have no system of checks and balances (like peer review) to keep them from thinking that smart=always right, get really wanky, really fast.

/tl;dr