@WildebeestH - you have forgotten another group - those who didn't go to grammar schools themselves, but know someone who did - parent, child, friend.
Two of my dses went to grammar schools before we moved, and I was impressed by the quality of the education they got - but following our move, they went to a very good comprehensive school, as did the youngest ds, and they all did very well there.
I am sure ds1 and ds2 would have done fine wherever they'd gone, but ds3 benefited from being at a comprehensive school - he had not passed the 11+ high enough to get into one of the grammar schools, and would have ended up in a not-particularly-good comprehensive, had we not moved, and got him into the good school with his brothers.
I went to a comprehensive school - it had previously been a secondary modern, and my year was the first fully comprehensive year. Had I been a year older, I would have done the 11+, and I am pretty sure I would have gone to the local grammar school. As I was bullied (partly for being academically smart and a hard worker) at the comprehensive, I am sure I would have been happier at the grammar school. But that chance was denied to me.