It depends what years you're in. I attended Pre Uni (Homebush) from Yr 3 - Yr 10 (I'm currently doing my HSC, so really, I'm procrastinating right now).
I personally think it's a really good start for primary school students, because... well... You actually learn. My school teacher in Year 4 was still trying to convince me that I didn't actually know how to write 1 - 100 in a 10 x 10 box grid. I very nearly threw my book at her.
The whole "Step-by-step" booklets aren't too bad in the junior years of high school either, although occasionally they mix up the science ones pretty badly, so really, it all comes down to which teacher you get.
The additional maths lessons are also pretty useful, as they employ uni students to tutor you for whatever you don't know, but it takes your own initiative to maximise their ability to help you.
I left, so I'm not sure how they are in Senior years, but overall they have a fairly good academic record so that has to count for something, right?