DISCLAIMER: I attend a grammar school myself, however, this post represents my views and are not those held by my schools or my friends or whatever. It is purely me speaking my mind.
Firstly, I believe that selection at 11 is wrong because the potential career paths of a student are sadly diminished based on the results of one test taken over four parts on one day. Before even having a thought about which door to go through, some of the doors are shut off, and that is just plainly wrong.
Secondly, the demographics are very distorted in grammar schools. In MY grammar school, if you are not a white, middle class student, you stick out like a sore thumb. In my year, there are 2 black girls and no black boys. THAT'S IT!! I used to live in the US, and half of my year was black (my year was 200 students strong). To me that kind of demographic distortion is ridiculous, and is another reason why grammars are a bad idea.
Third, it limits opportunity for the students stuck in comprehensives. Now I know that they have come a long way since secondary moderns, but the stats don't lie. Comprehensive students do worse in grammar areas than their equivalents in non-grammar areas, and that is because staff AND student expertise is sucked up by the more illustrious grammars. If smarter students (like me) were in a comprehensive, then we would still be able to succeed, but our attributes could trickle down to the less academically able students. In essence, with all things, academic ability would move towards equilibrium, or even up as a whole.
Also, rather than all this discussion about grammar this and reforms that, let's do what the Scandinavians do, INVEST!! The easiest, and by far most effective way to improve education and social mobility is to invest a shitstorm of money in the education system. What we've got right now (without grammars everywhere) is a good base, and can be improved greatly with a *****on of money.
Again, I reaffirm that this is me speaking my mind and giving some anecdotal evidence for my opinions