>>1
I had the opportunity to join (I guess I still could), but I didn't see a point to it. I like puzzles, but I can find plenty of those without Mensa (besides, IIRC Mensa loves word puzzles, and I prefer spatial puzzles). I don't need peers, and I've gotten so used to being alone that I enjoy it, so a social group wouldn't buy me much.
I've read things written by Mensa members, and they always come off as shallow and pretentious. Their understanding of the various topics they philosophize about appear fairly superficial, and seem to be governed by the "popular science" genre of books. I guess when you've been told that you're smart, and that everyone else around you is smart, you have to rush to say things perceived as "smart" to help you fit in (and thus don't have the time to internalize them and, you know, actually
think about them). If you are the type of person who finds Douglas Hofstadter fascinating, programs in Haskell because you think it makes you a math genius or constantly asks questions in class that you already know the answer to (to impress the teacher), Mensa will probably be a good fit for you.
If you don't care about the puzzles or the social interaction, it's really just a club full of people who sit around masturbating themselves over how smart they are. I've seen little action taken towards helping gifted people succeed. It's easier just to reassure yourself that you're "smart enough to do anything you want, if only you cared enough to do it." That's the illness I've had to spend most of my life curing, and I'm fairly certain I'd relapse if I allowed myself to join Mensa, so I don't plan on going anywhere near it.
Then again, maybe joining Mensa will boost your enthusiasm and give the motivation to do something great. It's up to you to decide.