TOSN,
We are really lucky, because that's exactly the choice that my children have had - they have passed for our (few) local grammars BUT we live in the catchment of an excellent comprehensive and both children have chosen that instead of the journey to a grammar.
However, we are in an exceptionally lucky position, as the comp in question comes above some of the grammars (and all but 1 of the privates) in terms of absolute results, and above almost all in terms of added value.
Given that the margin between 'success' and 'failure' at 11+ is tiny, the question about grammars vs comprehensives has to be about who they disadvantage as much as who they advantage.
At the very top of the distribution of 'academic ability', there are a very tiny number of children so able that they are genuinely difficult to educate in a system designed for the majority - and as I have said upthread, a system of 'special schools', for those whose ability is so high that mainstream schooling is both inappropriate and inefficient may be needed. A very few superselectives are probably already in this category - I did once work out that the one closest to me probably took 1 in 5-10,000 of the children in its 'effective catchment' - whether it effectively identifies the correct children is another matter; it probably just takes a random, heavily coached, selection of the top 5%.
Then there is the vast bulk of the 'middle ground'. The existing grammar school system in many areas - and the traditional grammar school system even more so - puts a line through these and puts some in a 'grammar' box and some in a 'secondary modern' box. However, on a different day, a very large number of children - at least the top set of any sec mod and the bottom of the grammar, possibly even 2 sets of each - could have been swapped over, as statistically there is no genuine difference between their intelligence. Yet the system pretends that these children 'belong' in different boxes, and give them separate educations with different opportunities. Which is at best unfair and at worst immorally wasteful, especially since the current employment landscape (much more than the 'blue collar / white collar' dichotomy of yesteryear, for which grammars were designed) requires very much the same of ALL these children when they gro to be adults - which is why a comprehensive system that gives them all the same opportunities makes so much more sense.
There is also the issue of 'spiky' children - exceptional mathematicians with ESL who flunk VR tests, for example. In a comprehensivem children like these are simply set for each subject appropriately. In a secondary modern, their true maths set is not available.....
Mobility between sets is, IME, relatively easy in the younger years of comprehensives: DS and his friends are re-assorted every half term.