I haven't taken a Manhattan course, but bought the SC guide to help with my preparation. I am extremely impressed with the quality of the Manhattan GMAT tests and how closely they represent the actual GMAT. The CAT algorithm is a cut above that of all the other testing companies and is second only to GMATPrep. The difficulty of the quantitative sections helped me to focus on my weaknesses and definitely played a part in improving my quantitative score. I am glad I purchased the Manhattan Guide and did the Manhattan tests.
Here is a breakdown of my scores on the practice tests:
MGMAT CAT 1 - 680 (Q48, V35)
MGMAT CAT 2 - 680 (Q48, V35)
MGMAT CAT 3 - 680 (Q43 - did not finish, left three questions unanswered, V39)
MGMAT CAT 4 - 720 (Q47, V42)
MGMAT CAT 5 - 790 (Q50, V51)
MGMAT CAT 6 - 800 (Q51, V51)
GMATPrep Test 1 - 750 (Q50, V - can't remember, don't know how to see the score again, but I got 8 questions wrong)
GMATPrep Test 2 - 770 (Q50, V47)