Arabic Online is a web-based course with native speaker audio, speaking practice, and recall activities. Although they provide a Guide To Writing Arabic e-book, you may want to check out Arabic Quick! for a more comprehensive introduction. This will help you at the intermediate level when most of the text uses the Arabic script.
The section on Advanced Arabic and the Grammar Explorer seem promising: they have interactive activities and texts to improve your grammar, sentence building, and reading comprehension. The rest of the courses, however, focus on memorizing phrases for talking about food, shopping, travel, and formal and informal meetings by listening to and replicating dialogues. There isn’t as much focus on building sentences as there is about memorizing them.
If you’re going to pay for a course, Arabic Online doesn’t seem as thorough or straightforward as other options. Additionally, the site has some bugs that can make navigation frustrating. Total beginners should probably look elsewhere to start their learning journey.