Many people are searching for online courses to learn the guitar because they are often very cheap compared with local instructors paid by the hour and other benefits. You could find a high quality course with many DVD's created by a world-class instructor who charges very little comparing to what your local teacher would be charging you by the hour. Even your local teacher could be very limited and would not teach you advanced lessons, so if you want to learn the guitar from beginner to expert, saving a lot of money in the process, the Internet is clearly the only way that makes sense.
Learning from a book is totally prehistoric. You will learn much more easily with software or DVD because of the interaction with the teacher. Although he wouldn't interact really with you, you could always pause the DVD to replicate things that where taught and learn at your own pace.
Not all people will feel comfortable taking the lessons from their computer, especially older people not as familiar with technology as younger people and because there is no interaction to ask questions and get immediate answers. Although some courses do have a members only forum in which you can get an answer fast if you post a question there (in some cases, even the creator of the course is an active member of this forum), for older people this process could seem a little "cold".
Playing guitar lesson in front of your PC is something weird for many people, but watching professionally mastered DVD's with detailed lessons, while drinking a cup of coffee is definitely much more comfortable. With either option, you wouldn't get embarrassed if you fail in any lesson because you only watch the DVD's or the software.
No doubt that one of the worst things about buying an online guitar course is the lack of many advanced lessons. This is normal because your local guitar instructor wouldn't teach advanced stuff without you paying hefty fees. Also, because demand is low for very advanced lessons you wouldn't find many quality instructors interested in teaching this kind of stuff.
Although few people will prefer "personalized" lessons and will not be comfortable learning from their PC or watching a DVD, most people are very satisfied with this kind of learning, as it shows the majority of posts left in one of the guitar learning courses available on the Internet. The interaction with other people in the forum is more or less the same as if these people were in the same classroom. The creator of the course himself participates actively in the forum, answering questions and adding content in the case of this specific course.