MGMAT tests are pretty accurate, especially since April 24th, the day when
MGMAT staff changed the scoring algorithm of
MGMAT tests to better reflect the scoring pattern of the actual GMAT test.
I found that
MGMAT's verbal is extremely close to the real thing, so if you ran out of time on verbal section, you need to practice your pacing.
MGMAT's quant questions are more difficult and time consuming than the actual GMAT's questions, however, the scoring is less punishing than that of the real GMAT, so at the end you should end up with a fairly good estimate of your raw math score. What is your V/Q breakdown?
MGMAT has a great tool that allows you to see the areas you are weak at, so definitely utilize it in your practice!
I suggest you take a GMATPrep test also.