CAT's

I had a fairly good experience with the Manhattan CAT's. They are a good indicator of where you stand. The questions are unambiguous and certainly of a higher difficulty level than those found in the official mocks. The explanations provided are comprehensive and easy to understand.
The only drawback was that the first three tests were much tougher than the actual GMAT and although i never scored more than 690 on the mocks i scored a 730 on the actual exam. However they did help me to get used to the pressure and acquaint myself with timing strategies. Overall i felt they were the closest representation of the exam and quite invaluable in my preparation.