I have used the grammar and usage parts of this site in my classes for two or three years, and I have also spent several hours exploring its resources. Its general title is accurate, because the site provides explanations, tutorials, and interactive exercises for a very wide range of grammar- and composition-related topics. The explanations use traditional schoolroom grammar and terminology, but they assume minimal prior knowledge and they are informed by a sophisticated understanding of how language works. Straight explanations are varied by PowerPoints, an "Ask Grammar" section, an FAQ, an extremely useful "Notorious Confusables" page, and various other resources.
The site's design makes it easy for instructors to give students URLs for specific grammatical concepts and usage rules to be reviewed (or learned for the first time). Clear explanations are followed by interactive exercises that illustrate the concepts and test students' understanding. My honors composition students frequently report not only that they really like the immediate feedback but also that the first interactive exercise makes them more attentive to material they thought they understood. Finding links to recommend is made simple by the site's search engine, index, and drop-down menus.
The pages of the site, including the interactive exercises, work well and nearly always seem to be available. The closest thing to a design problem I have encountered is having to retype, rather than cut and paste, when putting in a corrected sentence as an answer in some of the quizzes.
Those who know what they want to look up will find the site relatively easy to use, although its size and complexity can make it hard to be sure one has found the best available section for the purpose. My only reservation about the site's helpfulness for general users is that it may be hard to look up a rule if one does not know what an error is called--but that is a problem common to reference works on this subject.