I have taken them in the past. I think it depends on what the class is about, and who the teacher is. If its a bad teacher, its still a bad teacher. If its a good teacher it will show. In the past I have taken online classes for the following topics.
Japanese language. Horrible teacher,she made up words(my japanese friends had never heard them before.)
American sign language. Impossible! Very hard. Probley not near as hard if you have a teacher right there with you, but online it is incredably hard.
World religions. Teacher was very biased. Had good information, but since it was a morman teacher, she spent four times longer on that then the other religions.
History of western America. Only talked about washinton. not any of the other states.
so yeah, my exsperiounce with online classes wasn't very good. But as i said, it depends on the topic, and the teacher.
I much perfer to have a teacher right there, face to face, That way you can ask qeustions and get replys qwickly and the teacher can help you sort out any confusion you may have.