Imo, Golden Key is very much like "Who's Who of High School Students", "Who's Who of College Students", and "National Dean's List"... they were all just created to get your $$. I have thrown my invitations to each in the trash (besides the first year I was offered WWoHSS, because I was still naive) and have not regretted it for a minute.
Not to rain on your parade, PianoGirl, but the top 15% doesn't really seem that impressive; I would assume a very large number of premeds fall into that percentile. Besides, Adcoms can just look at your transcript to see your GPA.. they don't need you to list an honor society to know you did well.