I took Knewton course for my 2nd preparation and ended up with the same score as I got in my 1st attempt (660). Eventually, I claimed my money back.
This is not to say that Knewton is not good. In fact, Knewton has one of the best GMAT prep concepts and strategy. The problem is, as I have mentioned in a number of posts in BTG, the question style of real GMAT test, at least in my view, is nowhere like that of any practice tests from test prep companies.
Therefore, even though you can master the concepts and strategy given by many test prep companies, you don't want to get your self used to the questions style that you won't see on real GMAT test. In addition, you want to do AWA essays before going to Quant section of any practice tests. Many practice tests available are not packaged with essays, but that's okay. Essay topics are available for free in
www.mba.com; just pick any topics for argument and issue essays and spare 30 mins for each to compose well written responses before you start your practice tests.
Everything I said was merely derived from my personal experience. Make use of it as you feel comfortable.