They’re good if it’s a subject you’re strong in, or if you’re the type prefers reading a book to taking a class. If you’re weak on the subject though it’s a killer. My wife’s done a lot of online classes in subjects she was good in and breezed them, then she took macro-economics which she knew nothing about and it was a nightmare. Luckily for her owing to a lifetime hobby of FR like discussion I’m pretty solid on macro-economics and could steer her through until she got the core concepts locked in, but without some sort of teacher she would would have been doomed.
Although the subject and how it was presented may have been a big part of the problem, there are on-line courses where the students have direct interaction with the teacher. By using either audio or video conferencing technology some online schools offer what amount to regular classes. But instead of being in the room with the teacher you are watching and listening to them on your computer, and their blackboard is a window on your screen.
There are also schools which offer one on one tutoring on-line. For example, for a surprisingly small amount of money you can have a native speaker of a foreign language tutor you over a Skype call.
The same high speed network that enables Skype and Facetime also enables real time interaction with a remotely located teacher.