Useful as evidence of smarts; useful as a community of smart people. I was a member many years ago, just to see what it was like. Finding insufficient reason to stay, I left.

A community has to have some sort of focus, a reason for its members to be there, or it doesn't work as one. Being a bit brighter than the mass, and "enjoying each other's company and participating in a wide range of social and cultural activities" (from their web site) strikes me as rather diffuse. The company was, like Eliezer described, like a small SF convention -- but without the SF to provide the focus. I've been going to cons for a long time, but I only went to a few Mensa meetings.

When I was a member, I also went to a couple of AGMs, where intelligence was conspicuously not in evidence.