My son went to the school for 1 year and he had a great time! Personally, I wouldn't complain about the school except for the costly tuition. But they have a reason for this as they truly devote the attention on the needs of each individual student. My son has attention problem and it is indeed very hard to find the right level of special needs support in HK (even among other international schools!). There are many reasons for parents sending their children to The Harbour School, and generally speaking, it could be either children who require special needs support, or an "interim" school while waiting for another international school.
If you will enrol your kids because of the first reason, then you definitely choose the right school. The school is small, the infrastructure may not be fancy as other international schools - BUT their efforts and qualification to cater your child's special needs are the best in HK.
If you will enrol your kids because of the second reason, well, obviously this simple reason can explain everything why your child go there.
Regardless, I like the school a lot and thinks my son has benefited significantly during his one year there. Good luck!