I found these books extremly usefull. The best part is that you can purchase OG11 and
OG suplement books on the same website, which is very convenient especially for international students who can significantly reduce shipment costs by purchasing all books from one provider.
I haven't bought all books but ordered OG11,
OG suplements, 3 math books (out of 5) and SC book (so called SC Bible). I thought I don't need books "Word translations" (my biggest mistake) and "Geometry" because I felt pretty confident about geometry. After my order was placed I realised that behind a pretty stupid and innocent headline "Word translations" lies the toughest material: that book just starts with algebraic translations and then covers most advaced topics on the GMAT such as probability, combinatorics, work and rate problems etc.
So I placed new order and spent additional $45 for shipment costs. I bought "Geometry" as well, more because I didn't want my precious "Word translations" to travel alone across the globe.
IMO one should buy that 6 books: all math books and SC, or just buy math books that cover your weak areas and... What? SC, of course. It really is a bible, must have.
My two cents on this topic: don't buy
MGMAT RC and CR books. You should study this material just from
OG and GMATPrep because you are going to miss the target if you are learning from some other source. You are not entering the mind of GMAT question writers and you will be in the deep trouble both if you overshoot the question by getting used to some weird geeky assumptions (
MGMAT) and if you undershoot the target by using some chainsaw instruction manual such as Kaplan. Of course, that is just my oppinion.