Hard but is it accurate?

The math is definitely difficult. I feel confident saying that whatever math score you receive on Manhattan math is sure to be lower than what you will receive on the real test. As a result it is definitely good for practicing speed, fundamentals and word questions (which can be very tricky). Still if your goal is to get used to the format of the test, I think Manhattan is far off. The math questions don't match GMAT questions, often taking too long to solve and involving ambiguities in language.

The verbal section on the other hand is a bit easy. The reading section I think is the closest to the real test but the sentence corrections and critical reasoning are off. The sentence correction questions rely primarily on grammar mistakes with little attention given to nuance. The critical reasoning section follows standard patterns such as correlation equal causation and mistaken statistics (such as mixing up percent and raw numbers). As a result the verbal score on Manhattan could be much higher than that on the real test. Overall, not a bad practice test but the overall score is not reflective of what you will receive on the real test