I think that people who enjoyed a "good" comprehensive education don't really appreciate what a "bad" one looks like, whether in a failing school, or a school with bullying problems, or a school with other behaviour issues, etc.
Likewise people who went to a grammar and found it tough may not fully appreciate how tough at comp may have been too!
I was a straight A* pupil upon leaving primary school, but suffered a truly horrendous crap comp, and left aged 16 having failed all my GCSEs. I retook them partly at college and partly by self study and passed them all, then passed A levels by self study and then chartered accountancy exams by a combination of self study and evening classes.
I'd never wish a "crap comp" on my son, who was likewise bright at primary school, so he took the 11+ (with minimal tutoring from us), passed it, and got a full suite of 8/9 grades at GCSE and 4 A*s at A level, and went onto a First degree in Maths at Uni.
Some of his primary friends who were likewise on the "top table" at primary and went to comps have done a lot worse and talking to them and/or their parents (we live in a village), some had a pretty horrid time at the comp and really struggled - bullying, disruption, etc.
IF, and that's the crux, IF the child is a high performer generally, then I'd always go for the grammar option so their education isn't limited. However, if they're not high achievers, then a grammar probably isn't for them. A kind of equation really - the more "tutoring" they need, the less likely they're flourish at a grammar!