My son is in P1. From what I see, most, if not all of his classmates are very good in English and the PHW standard in HKUGA is tough. I would imagine, these kids should be very adaptive and should be able to do well in english secondary school with ease.
To me, HKUGA itself is good. What's also important is the location of the campus - being able to grow up in a relatively modest and nice/old district. I even enjoy go picking the kid up and go for a walk at the wet market or grab an egg tart on the street.
My girl also got an offer this year. She is very happy to be able to follow her brother to the same school.
No regret so far and heck, HKUGA way exceeds my expectation. It may not be the best school for everyone but it works great for me.