It’s essential to prepare for the GMAT with full-length computer adaptive tests (CATs), because the CAT format has lots of unique features (for example, the importance of skipped questions). Bottom line: you need to get your hands on some CATs and make sure that you practice wisely with them, because they are a relatively scarce resource.

Even to those (very wise) test takers who have enrolled in a comprehensive program of that kind, the two free GMATPrep® tests available at mba.com is a key part of practice.

Three things that make GMATPrep® tests special:

1.  They are created by GMAC, the creators of the GMAT itself.
2.  They include retired test questions from the actual GMAT. (The only other source to do that is the Official Guide, also by GMAC.)
3.  They give very accurate scores.

The tests can be downloaded here.

Drawbacks to the GMATPrep® tests:

1.  There are only two of them.
2.  They don’t offer explanations.
3.  They don’t provide diagnostic results.

A comprehensive GMAT course includes a battery of full-length practice tests that are enhanced by explanations and diagnostic results. So that addresses the drawbacks.

Take a GMATPrep® test when you want to have a really good idea of your current GMAT ability. You don’t need to spend $250 to take the GMAT and get an idea of your score (and partly for that reason, doing so may send a message to admissions officers that you aren’t approaching the b-school process seriously or competently).

Since there are only two GMATPrep® tests, you need to conserve them. Save at least one for the moment when you think you’re really really ready to go rock the test. A GMATPrep® test is a good reality check at that point – if you have a representative test experience (i.e., not too many distractions) when you take one and your score is much lower than you think it should be, you might want to reconsider your test schedule.

One final note: test takers have copied many of the GMATPrep® questions into forums for group discussions. There is certainly value to this behavior, since the tests don’t contain explanations, but it’s something my students have appreciated a heads-up about.