Bertrand Can I ask what you mean by that? Presumably you had GCSEs and A levels just like them?
I knew I would get asked about this. 
The difference was that, in our comp, we were taught the material we needed to know for the exams, but nothing more. By contrast, my grammar and private school peers at university had a far wider-ranging knowledge set and understanding of the subject.
A good way to describe it would be the way we were taught French. For five years, we were, more or less, drilled to regurgitate sentences for the GCSE. So although many of us got fairly good grades, we couldn't actually communicate in French beyond asking for a bunch of grapes or telling someone what our name was or where we lived. We were never taught, for example, how to conjugate the verb "to be" into the plural form in French. My private-schooled friends, however, were somewhere around B2 on the Common European Framework with only a GCSE. They could actually have a passable conversation with a French person in French.
The same went for English literature. I got extremely high grades, but we were, again, drilled on the set texts and never taught anything about how those texts fit into the larger literary canon or that, indeed, there was even a larger literary canon. My grammar school and privately-schooled peers were, however, taught all this.
So when I went to university, I spent my first and second year trying to catch up with what these students all already knew in this regard. It was an enormous amount of work. The schism between my knowledge at 18 and their knowledge was vast.
I would say I was schooled in an area at a time that could be described as the point where if the comprehensive system could work, it should have worked then. Everyone in my area, bar a few very wealthy families, sent their children to the local comprehensive. The nearest grammar wasn't really a consideration for anybody as you needed to drive your child to the school and it was a time when almost every family either only had one car that was solely for the use of a parent getting to work or didn't have a car at all.
Yet, despite a wide mix of children from different socio-economic backgrounds, it just didn't work. Nobody achieved their potential. It is notable that many of my former school mates spent their late teens, 20s and even 30s in further education to achieve what they should have done at 16 and 18. And these people were not "late developers"; they just were not taught properly or encouraged to have confidence or aspirations.
And I am not just talking about academic qualifications either. My comp stopped access to the car pit in the CDT workshop in my second year, and stopped the CDT lunchtime workshops as well. So all the lads that were interested in car mechanics, joinery or engineering found that route for further learning closed down to them.
To some extent, I agree with headofthe4hive when she says I think a lot of comprehensives are actually secondary moderns. This is certainly true in my experience, and that of my former schoolfriends with children going through the system, and that of my DH's friends.
One thing my comp experience (and my subsequent teaching experiences) did teach me, however, was that almost every child in Britain without significant learning difficulties will be very good at something that, in some way, maps back to an academic subject. It's very difficult to be a good joiner and not have a good, working grasp of geometry, for example.
To my mind, what we have is an old-fashioned, classical divide between academic and technical/craft/trade when really this divide is now a false dichotomy between theory and practice. But theory informs practice and visa versa. We need to recognise this, and develop our education system so we support the application of academic disciplines.
I would support a system where grammars and technical schools worked hand-in-hand with cross-fertilisation between the two institutions and pupil migration between the two systems.