Very useful

I think one sure way of getting a high score is by using Manhattan GMAT guides and regularly reading Ron Purewal's blogs. I learned a lot just by reading his blogs where he explains concepts with a lot of details.

The Manhattan GMAT books are comprehensive and I believe the verbal sections helped me a lot. I think the SC book is by far the best book in the market for cracking the SC questions in the actual exam.

The tests were pretty accurate and the explanations for the answers were useful. Though I think the tests are slightly harder than the real exam they were useful in that they motivated me to study even harder for the actual test! In the actual test I scored about 30 points more than the diagnostic test.