Some students start with the Official Guide and then look for something to supplement it. A lot of students look for a course that’s affordable, online and well-organised. For that reason, quite a few end up with Magoosh.
Student feedback on Magoosh is generally good: several of my students have been very happy with the videos, and they are nicely organised by topic so you know which bits of the GMAT curriculum you’re tackling. Magoosh also have flashcards, practice questions and tests, and some useful blog posts. One of their senior guys, Mike McGarry, clearly knows his way around the GMAT and has written some good stuff.
What I like about Magoosh is that everything is clearly laid out, but they’re not trying to dazzle you or sell you false promises. They’re also confident enough in their own materials to recommend other people’s, which to me is always a very good sign. Agencies or tutors that imply that everyone else is useless or inferior are usually trying to make up for a substandard product with an aggressive sales technique. Magoosh offer a 7-day free trial so you can see whether they can help.
Some caveats: their materials aren’t perfect, and although as mentioned the Quant material is ordered by topic, there are varying levels of difficulty within each topic, so you may find yourself jumping from easy to hard and back again, which isn’t ideal. Their 3-month study plan requires a ridiculously long list of resources and recommends that you do the Diagnostic Test from the Official Guide on Day 1 ~ terrible advice if you ask me. Some of their questions aren’t great (i.e. realistic or well-constructed), and some of their explanations miss things or say things that are just plain wrong. But these faults are relatively rare and usually unimportant.
You won’t go too far wrong with Magoosh. You can pick and choose from a wide array of GMAT materials, and you’ll certainly make progress. So it’s a thumbs up from me. Of course you could always try Chuck Dreyer’s GMAT Preparation; I hear he’s pretty good too…