NMAT might not be the best use of your time if your only objective is to get into top B-schools.  The primary attraction is NMIMS Mumbai, a good school that is not on par with IIM Ahmedabad, Bangalore, Calcutta, XLRI Jamshedpur, or FMS Delhi.  Colleges like KJ Somaiya (Mumbai), SDA Bocconi (Mumbai), TAPMI (Manipal), and a few international schools are accessible through NMAT in addition to NMIMS; they are all respectable but not in the "topmost" category.

 Hence, concentrate on CAT and XAT if you are only interested in the best universities.

True, NMAT is mostly for NMIMS but for me it’s a backup if CAT/XAT goes wrong.