I tend to view the grammar/secondary modern system as a historical relic since I haven't lived in an area that had this system since I was at primary school.
I was in the first year of comprehensive education in my area, and, tbh, my parents were too worried about me going to the ex-secondary modern - my catchment school - to send me there and I went to a private school instead, from a state primary.
It was quite a leap of faith for me, therefore, to send my children to (different) comprehensive state schools. But I'm very glad I did. They have both done/are doing very well and I've saved lots of money on school fees (though it would have been a stretch for us). Neither of them have been – or needed to be – tutored at any point. They've had great teachers and they have both learned a lot. The one who has done GCSEs did far better than I did and is now doing well at A-level.
I hear some concerns further up the thread about what happens to the academic children in a comprehensive school. In my experience, they find friends who are similar to them and also friends who are not similar to them but who they get on with and learn from – and may not have met in a selective school.
There are kids who mess around and don't do well. (No school libraries burnt down though!) But who those kids are is not necessarily predicted by class or family finances. Issues such as SEN and children who are stressed by whatever's going on at home also probably play a role in this.
The grammar school/secondary modern system doesn't work because it writes off too many of the kids who go to the secondary modern and who may have done very well in a single comprehensive school. But the comprehensive system probably does need people from all backgrounds to send their kids there in order for it to work as well as it could do. I think the comprehensives my kids have gone to probably have achieved that very well.