I actually wrote and edited a lot of
Magoosh’s GMAT content. I no longer work for them, but left on good terms. So hopefully you’ll find my opinion objective and trustworthy. I would say the videos alone are good, but not quite enough. You’ll also want to go through a lot of practice questions and answer explanations. If you don’t have access to
Magoosh practice questions, use the free official ones from GMAC, and seek out answer explanations by searching for the questions here on GMAT Club.
If you have access to
Magoosh’s videos and their practice quesitons and explanaitons, I would still recommend using additional free official practice questions. No matter how good mock quesitons are (and I think the quesitons I helped create for
Magoosh are pretty good), nothing is quite as good as the official questions, which come from real past exams.
You’ll also want to build general acadmeic skills beyodn the test if possible. Look up some good video or written tutorials on different math skills as well as critical reading and grammar skills and use that as a supplement to any GMAT-specific learning material.
Hope this helps.