As a parent that has a son that has been studying a foreign language, one to one, on line for the past two years I have to say that this has been a very positive experience for us. I would say that it is not as effective as a teacher sitting next to my son, but there are a number of positives, which for us far outweigh this negative.
Positives being:
It is much easier to find an online teacher, than a teacher that is going to come to our house.
It is so easy to schedule lessons- This may sound like a small positive, but it really is a massive positive for us. When we go on holiday my son usually still has his lessons. For all we know the tutor may well be on holiday as well. Very few lessons cancelled. If they are cancelled then because there is no travelling time involved they are much easier to reschedule. I believe this can mean a lot more lessons can be taught and therefore the tutor can earn more money. My son is scheduled to have 2 half hour lessons per week and despite us often being away he often requests additional lessons and he has had about 120 lessons per year for the last two years as he has requested extra lessons, during holidays, etc.
Online lessons are often cheaper because: you have more teachers to chose from (primarily because you are not restricted by their geographical location) and they are less time consuming for the teacher if it means they don't have to travel.
Teachers are more flexible about lesson durations- I.E. "How about sticking another 30 lesson in tonight at 6:30pm?" Doubt many tutors would like to come to our house for a 30 minute lesson, at short notice.
Safe guarding issues-these lessons can be done via a site, such as Teachy.com (never used them, but it sounds like they have interesting technology), where they have a system similar to Skype, where the video calls are recorded and can be played back, if necessary. I would imagine this could be positive for a dedicated student and would keep the tutor on their toes.
Summary
As a parent with a son studying a non core MFL online I can say this has been a great success for us, primarily because we can find a teacher more easily and it is much easier to schedule lessons, but as stated above there are other positives as well.
I am thinking that my son may well be tutored in another foreign language when his level in that language improves a little and I can't imagine this not being online.