I went to a grammar: we went faster than the local comprehensives/6th form colleges(and were aware of that!), had more homework, and didn't have access to vocational courses.
The emphasis was on academia and academic achievement, as well as all-round achievement (sport, music etc) as an accessory to that. We were given the opportunity to do everything from debating teams, athletics, etc and you could take somet ime out of lessons to do those things.
Sometimes, it was a bit too exam-focussed. In 6th form, we had to go to 80% of lessons to be eligible for the exam. I went to approx 40% in English (not proud of this now!) and was still put forward, because I was a 'cert' for an A. So in that sense, sometimes they just banked on the grade rather than nurturing the pupil. Exceptional cases, however.
However, it was the perfect place for motivated, academic girls. We got to do Advanced Extension Awards, and the teachers encouraged wider reading, arguing, interest in politics, debate etc. People came out motivated, confident and sparky.
For someone that couldn' keep up, it would have been a miserable place however. I was crap at maths, couldn't keep up, and it haunted me until GCSE's were over, because they didn't slow down and wait.
Still, would certainly encourage DC to apply if he/she seemed to have the temperament and academic ability.