I would second that. I think Knewton does a good job of the verbal section (disclaimer: I am a Knewton student). If I had to compare the verbal section of the
Manhattan GMAT (from the test it provides with the Guides) with Knewton's I think Knewton's is more representative of the real GMAT.
In addition, I think you should get yourself the Manhattan SC Guide. It is by far the best book to improve your score/hit rate in the sentence correction section. For Critical Reasoning I would say go with the Powerscore CR Bible.
If you can afford it and do decide to take a Knewton course, I can refer you and we'll both end up making $100!