I've joined and quit Mensa twice. The first time I didn't go to any meetings, because they looked too lively for me, but I did read the newsletter and was put off by a column by two people arguing with each other for the sake of arguing. So I quit the first year. That was when I was 26.
When I was 34 I rejoined for a couple of years and went to a few things. Most people were very nice, but there were a few obnoxious people who went to everything, and I had a hard time avoiding having to talk to them, which I found stressful. That was one reason I quit the second time. Another was cost (I was back on welfare by this point and found having to pay both a membership and the cost of lunch out too much). The third reason I requit was because I was having issues around giftedness as a curse and no one seemed really interested in talking about that, so I joined Triple Nine Society instead and got some good information and support there (for a while).
What Mensa would have been really good for (and the reason I rejoined) was job hunting. If you want to find a job in a geek-friendly company, you can ask around Mensa. I found a company where three of the lunch regulars worked, and they were expanding (it was a very small new company that would have been hard to find otherwise). I didn't apply for work there because my instincts told me not to, but I did have a very good interview with someone there who gave me some perspective on what employers are looking for.
Mensa is probably pretty good for meeting people and networking. If you're lucky you'll make some friends, too. You just have to enjoy doing the things other people do (e.g. going for lunch).