I believe pros and cons of learning a language online should be considered case by case and not as a generalization. I´ve had students of Spanish who wouldn´t dream of going online because of their poor computer skills (nothing that a bit of practice and good virtual classroom habits couldn´t solve) and I had some others that couldn´t wait to engage in the experience. Working on the web can provide high motivation to some, adding to the initial motivation to learn Spanish… and be unthinkable to others!
I agree with Ricardo that books and CDS are a complement to learning, specially with elementary and pre-elementary students but not necessarily the key to success in learning. I also think that having the resources (bandwidth, a running computer, your teacher and yourself) and the drive to learn, online learning has more pros than cons compared to face-to-face lessons, both for the teacher and the students. Saving time, money, schedule flexibility, having all resources in the same place and time, class recordings to watch again and again, better organization and the comfort of working from home against the occasional blackout or ISP problem.