MGMAT Quant will be slightly tougher and lengthier in calculations/math than actual GMAT. Dont expect too much delta, but if you do well in MGMAT quant then you will do better in actual GMAT.

Also MGMAT has recently re-calibrated their scoring resulting in approx 18 points lower score than earlier.

See below:
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CAT Recalibration – August 29, 2012
We have recalibrated our CAT scoring. CATs taken AFTER 11:00 AM EDT on August 29, 2012 are scored according to the new calibration. Older CAT scores are unaffected.
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Why are scores being recalibrated?
We periodically recalibrate to have our CAT scores correspond to actual GMAT scores as closely as possible. In addition, the percentiles in our system now match the latest percentiles published by GMAC (earlier this month). These percentiles, especially on the quant side, are undergoing significant shifts.

What is the effect of the recalibration?
New MGMAT Quant subscores, on average, are lower by approximately 1 point.
New MGMAT Verbal subscores, on average, are lower by approximately 1-2 points.
New MGMAT Overall scores, on average, are lower by approximately 18 points.*

What does this mean for me?
When you compare older practice scores to practice scores generated after noon on August 29th, you should subtract 1-2 points from each old subscore and about 15-20 points from the old overall score, to get a rough sense of how you would have done under the new scoring.
We know that this is a significant recalibration, one that impacts your sense of your progress. We’re committed to making our CATs ever better over time.

*Note: An earlier measurement of the effect of the recalibration overestimated the average impact on subscores by 1-2 points and on overall scores by ~9 points.
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Thanks,
PraPon

VOTE: vote-best-gmat-practice-tests-excluding-gmatprep-144859.html
Tough RCs: Passage1 | Passage2 | Passage3 | Passage4 | Passage5 | Passage6 | Passage7

I think MGMAT tests are the best source for relevant overall practising and preparation.
GMAT Club tests are clearly the best way to improve Math. Kaplan way too difficult, but good for additional practice. Knewton looks nice, but my experience of intensive studying with their tests and questions during entire month showed slight improvement in Math and drop in Verbal (from 710 to 660). My opinion - Knewton is good, have difficult questions, but irreleveant

Absolutely true, Kaplan scoring is not accurate. At least old Kaplan tests give low score even if your knowledge is good. Not sure, however, about current on-line Kaplan tests.

AHHHHHH! I got a 710, 690, 700, 710 on my practice tests taken under real conditions and then just got a 640 on the test. Wtf???

Any Reviews on Grockit tests haven't heard much about them?

Manhattan probably offers the best overall practice test experience from the above list. GMATClub tests are unquestionably powerful for improving one's quant skills, but I used those strictly as question practice rather than a full CAT (math plus verbal).