I purchased GMATclub premium membership along with the tests, just 3 weeks before my first GMAT attempt. It is one of the best investments I have made for GMAT preparation. By far GMATClub has the most difficult 700 level questions than any other test prep company. The bookmarking feature during the mock test is very helpful to review the error logs. With the premium membership, I was able to purchase the GMATclub app from the Appstore. The app was very helpful for me. I was able to practice SC and CR questions from my iPad/iPhone while I was travelling for work. A ton of free resources are provided with premium membership. OG Guide + GMATClub will definitely take you to 45+ in Quant.