The GMATPrep software is the best replicator of the actual test experience. It's also the most accurate indicator of one's level. It's important, though, to always take practice CAT's under realistic test conditions in order to extract the most benefit from them.
And yes, the Manhattan tests have a reputation for their difficulty. But these CAT's are very good practice and have decent accuracy, in terms of performance estimation.
Get the best GMAT Prep Resources with GMAT Club Premium Membership