I live in a wholly selective county. There are no comprehensives, only grammars and secondaries.
There are very few secondaries that are decent - outweighed by the crap ones. There is a very high demand for them, so unless you live on top of them/are very religious, you face your child going to a crap secondary.
There is a large cohort at the grammars (particularly the single sex ones), who would have gone to public schools if they hadn't passed, which shows how highly regarded the schools are.
It is still a great leveller though. Nearly all are tutored. My two children (one of whom was a child to a single mum on benefits, went to a decent primary though - my youngest went to a school that was dire and put into special measures), weren't tutored.
I didn't pass my 12+ (as it was then), in fact I didn't even take on of the papers as I was always sick. I suffered from school phobia. I still went on to do A levels and went to university, but it wasn't the success or failure of the selective process that helped me.
Children are all different, it's nurturing the child, not trying to get the child to fit into a hole, that is important.