Mensa only does testing for ages 14 and up and they are prohibited from giving out anything that says, "this is your IQ." The test is similar to an IQ test, but since it isn't individually administered by a psychologist, it cannot be used as an IQ number for admission to other programs usually.
I am a Mensa member and one of the gifted youth coordinators for my local group. I've met some people who I really enjoy through Mensa, but I'd say that the experience varies depending on the demographics of your local group and the population density of the area covered by your group. We've never been able to get a good regular kids program going with my local group nor have we had any regular local meetings in my area in quite a while. My local group covers a region that is hundreds of miles and includes some rather rural areas, so we're all a bit spread out.
If I could afford it, I'd go to the Annual Gathering every year, though. I loved the two I was able to attend in years past. One of them was the "world gathering" so it had some pretty diverse representation. The thing I've liked the best about Mensa, and I know that this is going to sound weird given that is an exclusive group by nature, is that is is very inclusive. Every member is welcome regardless of whether you are a cranky old guy or a transvestite belly dancer.