Tests are really helpful.

The test interface is very user friendly, allowing the tester to choose the topics of one's choice. Also, post test analysis is very detailed and crisp.
Apart from choosing of the topics, one can also select the level of questions - 500, 600 or 700 level. It really helps to pace up the preparation.
The option to choose the number of questions in a test also is a very helpful one.
Though, on extensive usage of the tests, one might fall short of the number of questions available, every question is linked to the discussions on it in the GMAT portal and this has links to other similar questions. So, this brings in a whole new set of questions.