I would recommend getting in contact with your local group and trying to meet a few members. What you get out of it could depend largely on how active your local group is and whether or not it's one you fit into. My local group is made up primarily of a much older crowd so I don't get much out of it besides a subscription to the Mensa magazine and local newsletter. There's a very active GenX/GenY Yahoo group, but it tends to be dominated by a handful of right-wing trolls. No thanks.

You need to coordinate a time to take the admission test through a local member anyway, assuming you haven't taken another test that could qualify you. There are quite a few that can be accepted so you'll want to look into it. It's possible you've already scored high enough on one of their accepted tests and you only need to submit the results.

If you don't anticipate being active in your local group you could go ahead and join anyway just to put it on a resume, although there are mixed opinions on how that's perceived by potential employers. I mention on my resume that I'm a member and I don't think it's ever been a problem, but I've heard about managers who toss resumes in the trash for it. As I write this I see that another commenter has said that they roll their eyes when someone mentions being a member. The reality probably is that very few people will be impressed by it.

The admission test itself is surprisingly simple. There are actually two of them and you only need to pass one. The first test is the Wonderlic and the second is a test designed by Mensa. If you fail both you don't get to reapply. You never learn your score. You're only told whether you passed or failed.

The tests contain nothing like those ridiculous Mensa brain teaser books. They're the types of questions that most reasonably intelligent people can answer fairly easily. They're not unlike the questions you see on the online Mensa practice test or one of those tickle.com IQ tests. What makes it difficult is that you've got insane time constraints. You end up with about 12-15 seconds per question. One test is fill in the blank and the other is multiple guess scantron. You basically have to be writing your answer while reading and solving the next problem. The person who proctored my exam said that there's about a 60% failure rate. It probably says more about the people applying than the actual test, but I thought it was interesting.

Is it worth it? I don't know. I don't get much out of the membership. I was interested in meeting up with similar minded people to have discussions, etc., but I think more than anything it was a personal challenge. I wanted to know if I could do it. As for the interaction with smart people, I think I get more out of HN than I could ever get out of Mensa.