I have done lots of studying by correspondence and achieved my chemistry degree through the Open University. I totally recommend this method of study EXCEPT when it comes to my PPL. Initially I thought I would study the ground school material at home and I'm fairly certain I would pass the exams using this method. BUT! (and this is a big but), the potential to tap into the experiences and knowledge of fellow student pilots and particularly instructors is well worth paying for. Fellow students will ask and get answers to questions you may never have thought of asking yourself. Sometimes the best information can come from the informal discussions in a group situation ( moderated by an instructor of course).