I didn't go to a grammar but am firmly in favour of them. I suffered a horrible crap comp where I started as a straight A student and left with only 1 GCSE. I just wasn't stretched at all, so I just got bored. Added to that some pretty horrendous bullying. The school was ex-grammar, we were the second comp intake, so at first, the older pupils were grammar, but as years passed, it was clear the school was going downhill fast due to behaviour, vandalism, etc. I vowed not to send any child of mine to that kind of toxic environment.
As it is, my son goes to a grammar and he absolutely loves it and is doing remarkably well (far better than we'd ever have thought), in fact he's in the top 15 of his year. He's being challenged and has risen to that challenge and more. He tells us that there's no bullying problem and he's made loads of good friends. Added to that, it's a boy's only school which he also likes as they aren't showing off to impress the girls, so can concentrate on what they're there for, i.e. education!
I'm a firm supporter of grammars. There was nothing wrong with them 50 years and nothing wrong with them today. The problem was that the old secondaries/sec mods/techs weren't adequate. But rather than improve the secs, they scrapped the whole lot. Grammars are perfectly fine, as long as the alternatives are also fit for purpose. In my opinion and experience, one size doesn't fit all!!!