Basically, SET is a secret society for highly gifted kids. So secretive, those who are in SET rarely talk about it.
Kids can get in by taking SAT test before 13 year old. If verbal or math score is higher than 700, they will be invited in. Most kids who get in are math/science type.
As for what the tangible benefit of SET, not too much. You get some free magazines and newsletters with stories of other overachieving kids. They are also invited to participated in certain restricted website such as cogito.org. SET members have their own chat room where occasionally they talk about nerdy things.
If your kid plan to take SAT test in 7th grade as many do for CTY and get above 700, then that is great. You get SET for free. But I don't think it worth the trouble of hiring a tutor for SAT prep at that age.
here is an article about SET. It is a bit old, but best reference that I can find.
http://www.davidsongifted.org/db/Articles_id_10035.aspx
