In my experience, Manhattan Quant are quality material, just not accurate.
I scored 49 on my real test Quant yet I never scored over 45 in Manhattan Quant. Primarily, because since most of the questions on Manhattan Quant are 600 and 700 level, you need more time to do them. I would always run out of time to finish up the quant section. But I also ended up scoring not too bad (despite missing 2-3 questions) since the difficulty level was high from almost the start. Some of the questions were slightly out of scope.

I strongly suggest taking GMAC FOCUS (set of quant tests). There might be repeat problems from OG but the questions are timed and they helped me raise my Quant score. Make sure you understand every question you missed during the review.