Most people consider GMATPrep to be the closest representation to the real test.
That being said, some people still crash and burn on exam day even when he/she consistently scores 700+ on practice tests.
I had the opposite experience. I never broke 680 on any practice test (Kaplan,
MGMAT or GMATPrep) and scored 710 on the real thing.
The main use of GMATprep is:
Real GMAT CAT feel (window, question layout, on screen toggle/radio button layout, etc.)
Real questions that are now retired GMAT questions
App developed by the same people who maintain the GMAT
Many of the people who post detailed debriefs are +/- 20 between GMATPrep and the real thing.