srl,
From our experience of sending 3 of ours there in recent times (Kowloon Junior followed by King George V in '94-'98), ESF schools are really very good. The standards easily match or surpass those of UK state schools. The kids are worked hard and a lot of homework is set at an early age. The schools are equipped to an excellent standard. Our kids went to the top of the class in many subjects when they returned to UK schooling 3 years ago. Discipline was excellent and the kids respected the staff. Staff morale was higher than we have found on our return, perhaps not surprising as they do get paid a more decent salary in HK.
I would say you would be more than pleased with the results of sending your child there. I wish mine were back there!