As someone who went to a grammar school, they are a terrible idea and comprehensive schools are a better system.

Students are not equally capable across all subjects, and their ability changes over time. Grammar schools mean there is no room to give you what you need in subjects you fall behind on, and students who start to struggle or start achieving post-11-plus have to transfer schools to fix it, creating huge friction and basically ensuring they'll miss out on the education they should have.

Comprehensives that have a full range of sets to teach at the skill level of the student for each subject are infinitely better for actual education.

I was one of the fortunate ones who was pretty generalist and so I didn't suffer too much by it, but I consistently saw people just give up on subjects because they were too far behind and the school had no other options because there were no lower sets.