I went to a Grammar School, 11 years since I completed GCSEs though so I don't know how much things have changed.
I was at secondary the same time as all of my cousins, 7 of them split between two different high schools.
I would say there was negatives and positives to both choices: my school would move through the curriculum at a faster pace so we had more time to go back on things right before end of year exams and have everything relatively fresh in our minds, whereas I remember a few of my cousins panicking for GCSE exams as they couldn't remember too much from things they'd been taught at the beginning of the year.
Also we could work to higher exam levels e.g. our top sets in maths took the Higher paper for GCSE (grades were A-C or a fail) and the bottom sets took the Intermediate paper (grades achievable were B-D or a fail), whereas the other school had top sets take Intermediate or Foundation (grades were C-F), I think one of the schools had a couple of pupils they considered to be exceptional so these kids took the Higher paper but still only got a C as the school didn't have the time or the staff with the skills to teach the children anything more than Intermediate level.
Also both the other schools allowed pupils to drop subjects in favour of taking an NVQ or foundation BTEC course in things such as hair and beauty or culinary or motor mechanics etc. whereas my school didn't give that as an option as it was expected that we go the more academic route.
I think the Grammar school was much more suited to the academic children and those who struggled to keep up just got left behind, whereas I think the other schools couldn't keep up with the more academic children so those kids weren't able to reach full potential in a non Grammar environment.
As I say, things may have changed, I've certainly noticed changes in schools now my sisters are in secondary, although they now live in a different area to where I grew up; 11+ doesn't happen and no Grammars in the area.