Has anyone used this series? Do you think they're useful/helpful books?

I've used the green Grade 6 one, primarily bought because I saw it had my favourite Heller study in it, and found it ok although not played every single one, mainly because I'm not very disciplined at the moment at playing things at the piano that I don't like. But I was just wondering if people do feel that they are a useful general technique-building series.

Having passed my Grade 5 in June, I'm now preparing Grade 6, the pieces for which I already know and 'only' have to be polished up (which usually takes longer than just learning the notes).
I think it'd be a good idea to do the studies from the ABRSM collection and I'm planning to do quite a few of them as those studies are supposed to contain the difficulties they claim should be tackled for the grade. I've got both series and I'll ask my teacher to pick and choose from both so we can do some over next year. An average of 5 per book should be enough and that may take ages to study. At a first glance, my teacher said they were really challenging for a student at my level.