I went too Haileybury many many years ago. I was not particular like the school while I was there however I did make many good friends whom I still see them regularly. Many things I did not like back then. Pastoral care was not great and the food was sucked.
It was not an academic hot house but 80-90% of the pupils still got to Russell group universities ( I got an update from the school and it still perform similarly to what it did back then). Oxbridge’s bridge number is between 5-12% each year. Most of the people are from around Hertford/ London area.
It has a great alumni network since in the past it has higher profile than today ( it trained and sent pupils to become a civil servant in India), Alan Sainsbury went there so as Clement Attlee (it is a strange place since it is more of Labour public school while the rest of public schools that are more conservative). Very strong link with the army and has one of the best CCF program in the country.
People who did bad in academic back then now doing very well in life. I think the school’s environment helped you to develop certain character ( confidence, grit and leadership) that is very helpful further on in life.
Academically going to this kind of school will get your child to Russell group university if your child are in the top 80-90 %, numbers are similar to those of Oundle, Rugby and Charterhouse. Even if your DC goes to Eton, Winchester, Sevenoaks, CLC or Wykeham abbey, if they are not in the top 35% it is most unlikely for them to get into Oxbridge and will attend to the same university as those in Haileybury or in other public schools anyway. In reality however it is not purely academic getting you into university, attitude and character are also important.
Academic is not everything I think it is important to look into what your DC enjoy. If your DC love CCF, Rugby and Hockey this is the place. If your DC prefer music than it is better to go to King’s school Canterbury where musical people are cooler and not branded as loser. However if you compare to school around Hertfordshire this is possibly the best., however if you have day option
Haberdashers' Aske's Boys' School is may be better but it is a lot more stressful.