Excellent, thorough, and lots of opportunities for bonuses!

As someone who hadn't thought about grammar or geometry since high school, I really needed a refresher. Manhattan GMAT gives the appropriate structure and great teaching (take Isaac's class!). One thing I'd say is to do ALL the work. None of it should be optional.
The online tools and course materials were extremely thorough. The most useful tool was the ability to analyze your test results by question difficulty and topic - really helped me target my weakenesses. I also felt the class prepared me mentally for the test - not just content.
The post-course assessment was also extremely helpful in determining my plan to take the test based on my previous exams. I felt discouraged because I hadn't broken past 670 on any diagnostic, but my instructor encouraged me to keep studying and I ended up getting a solid score on test day.