As others have said, Aylesbury Grammar suits a certain kind of boy. There is also the issue that they get next to no funding from the government, so everything is stretched and parents and alumni will be expected to make up the shortfall.
It's a very large school, so if you have come from a small primary or prep school then you run the risk of getting 'lost. There isn't the resource to give much pastoral care or assistance. Bullying is very common, and isn't dealt with satisfactorily in my opinion. For example, my son was bullied continually and reported it on numerous occasions, but it was never logged or acted upon. When he left and I cited the bullying, they seemed really surprised, and said they had no record of anything like that occurring, and could I please give them some evidence. I should add - bullying was from teachers as much as pupils. It will take a concerted effort to stop it, and I don't think the school has the resources or inclination at the moment - their attitude seems to be that it's what boys do.
It's a school for those of average to medium high intelligence, but not for the very bright. Many entrants failed the 11+ but got in on appeal, I have been told. In my son's year, all the brightest children ended up leaving at the end of year 8 and going to other schools. This is partly because they don't have the resources, but also they have the attitude that 'everyone at AGS is bright, so they should all be treated equally'. This attitude doesn't help bring out the best in children, and can lead to frustration if you want to do well in a subject, but can't. There was talk about setting up a GIFT stream, but I think there was resistance from some teachers. Examples of this include fielding rugby teams made from random players rather than having a first XI; any player could get picked, but then they always lose because other schools pick their 'best' players for the first team. Or, putting a very gifted mathematician into the bottom stream for maths because 'all boys at AGS are good at maths'. Or, for those who have already done their grade 5 music theory, giving them no tuition at all but just putting them in a corner with a grade 5 theory past paper.
Don't be fooled by the long list of clubs on the website, as most of them don't actually run, and many are unsupervised so it's just a room of boys messing about.
I'd say that AGS is a good state school but not exceptional. To a certain extent they are living off the historical reputation of grammar schools, but being so under-funded hampers their ability to do a good job. There is high staff turnover, and they can't attract the best teachers. There are no other decent schools in the area though, so you don't have much choice unless you are in a position to pay to go privately.