As for me, online learning is quite useful. You can study in any convenient place: on your favorite sofa, during lunch breaks and free minutes, outdoors in the park or forest in good weather. It's easy to get in touch with teachers at almost any time. Chats, various messengers, email, audio or video, forums, social networks are used to send completed tasks or clarify and clear points. By the way most of necessary materials provided by teachers and the rest can be easily found online.
Regarding the cons I think many people find it so difficult to force themselves to study in their free time and this is extremely problematic for teenagers. Not everyone can focus on learning information especially if it's provided in the form of online broadcasts or video lectures like in real time. If a student doesn't hear something he may miss important moments. Assessments based on test results don't always deserved. Students can answer questions with someone's help or search for information on the internet.
So I think online learning is pretty good but it also has disadvantages.