I've always fancied joining but assumed that I would never qualify and to be honest, having sat a few IQ tests in my time I would consider that to be an example of a pretty easy quiz.
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.
I did not go to one.