First of all, I love technology. I love the Internet, learning online and I love Skype. I taught English as a Second language on Skype for several years. That being said, I don't like to teach guitar over Skype. There are a lot of guitar teachers who, in order to gain more students, offer gutiar lessons using a web cam. I personally believe that many of these people are not qualified guitar teachers.
There are also several problems that can arise from teaching music online. One of the biggest problems is the inability for the teacher and student to play together due to the delay. I play guitar with my students. I play lead while the student accompanies me and then we switch off. Learning to play with other musicians is very important for students since most of them have a goal to play with other musicians in the future. I just received a new student last month who told me that he had been taking online lessons. Now he's driving for 20 minutes to reach my house, because he knows he can only get so far with online lessons.
I've had students with tempo problems; they have problems keeping a beat. One of the ways I help students to deal with timing problems is to count while they are playing, then I have them count while I'm playing. It's very effective, but impossible when learning over a webcam.
One other thing I don't care for with online lessons is the lack of personal contact. Modern society spends far too much time in front of screens already. They spend every moment in front of their TVs, cellphones and laptops. Do you really want your child to spend more time in front of a computer screen or do you want them to be involved in activities in the real world?