I don't inherently disagree with the concept of segregated education - those who want to learn can do so without the idiots at the back - but I think the selection criteria used in grammar schools are a joke, and remove the ability of children to progress.
At 11, it is not possible to accurately determine much of anything about a child's current or future abilities. What you're testing is a child's ability to successfully negotiate one test at one point in time, irrelevant of their background, their experience with similar tests, stress, levels of coaching, etc. There is undoubtedly a correlation between IQ at a young age and IQ (and 'success', however you define it) in later life - this is well studied, and a large chunk of it is genetic - but there should be a system in place that allows for progression of children according to their real abilities. In reality, this is already done through 'sets' within year groups for certain subjects like Maths, and is something that should be more widely applied, in my opinion.
I personally failed the 11+, but went on to have excellent GCSEs and A levels, went to Oxford, and have a PhD. Did the system work?