Great lesson materials and teachers
I've taken nearly 100 Flexi online classes in Mandarin at the HSK5+/6 levels. Each class has its own teaching materials (new words, grammar, fun side notes, etc.), which are exceptional and a major advantage over their competitors. The materials are more tailored than any textbook or class I've come across and a big reason why I continue to take the classes. The teachers are generally excellent too, and because the materials are so strong, even when the teaching quality varies the lessons are still successful. The group classes are also great value for money. Despite being "group" classes, the class sizes are very small, and many times I was the only student.