I started my GMAT Prep with Magoosh Premium Course. I was surprised by the clarity of concepts presented in the video's. I used Magoosh for both Quants and Verbal. I would say Magoosh's Quants videos can be considered among one of the best in the market, but verbal needs some work. One of the best advantage is the number of questions and the detailed video explanations for the questions. They have around 500 Questions in their database and they are really good.
The advantages i found with Magoosh are listed below:
Very good Quant
Customizable Study Plans
Amazing Customer Support [One of the best i have seen in market]
Overall i would recommend Magoosh to any one who are at the start of their preparation.