I do not know what sources people are using to form their opinions, but they are either completely wrong, or deliberately antagonistic, and if you honestly wish to know more about the centre or Dr Ngo then you should stop relying on these people.
I am an employee at Ngo and Sons. I am paid $80 to teach 1hr 45 mins, receive 9% superannuation, worker's compensation, and my two younger siblings receive free tuition. As you can see, this is definitely not cheap (it works out to be roughly $200 per week) and is much, much more than any other tutoring centres can boast, and even some other jobs, especially for young employees. He does not exploit us, ever. However this is not the reason the staff at Ngo and Sons stay there. Dr Ngo is an excellent employer and person who has given everybody around him so much and personally, it really saddens me to read some of the ignorant or misinformed comments on this thread.
Yes, some of our English teachers are from selective schools - they have considerable skill and experience (I see Ms Diab is quite famous!) - but we also have ex-students teaching English, and student and parent feedback has been encouraging. In particular, these ex-students are actually in the midst of their Education degrees (I know two are majoring in English too), so are receiving the newest training and resources too. Surprisingly, the drop-out rates for their classes are even lower than those belonging to some of the more experienced teachers.
I am not fresh from my HSC. In fact, I'm graduating from Uni soon. However all the teaching staff receive regular training by Dr Ngo to refresh our skills and get pointers on how to teach effectively, and ensure that our kids are getting the best we can give them. I can look you straight in the eye and tell you honestly that it is not just a business and not just maths - we genuinely care about our students. (By the way, thank you to my students who told me about this and have stuck up for the centre and Sir. But no, you're not getting extra marks in your exams!)