I also have to recommend going to a local group meeting. In my area, hopefuls can go to one or two meetings for a small fee to check it out. Going to a meeting as a hopeful definitely opened my eyes - the crowd wasn't what I expected at all.

That being said, I joined and never went to a meeting as a member. I used the Mensa Member tagline on my resume for a few years until I had enough experience that it wouldn't make a difference.