I have known about the audio go lessons for a while, and took a few lessons a couple months ago. Recently I decided to get more involved in studying this way. The site is run by Guo Juan 5p, and she has a number of other professionals who prepare lectures for her site. If you have a KGS Plus membership you will recognize the interface.
Since my opening is generally considered to be pretty good (thanks to lessons and workshops with Yilun Yang 7p) I decided to work on my middle game through the audio lessons. The web site has the middle game training series completed for the C level, which is 30k through 10k. In the past few days I have worked my way through all five level C middle game lectures. I have enjoyed working through the lectures and intend to view them each two more times before the time expires. You pay one Euro per lecture and you are allowed to view it as often as you like for a month.
I would recomend anyone who is not yet ready for private lessons and looking for an ecomonical way to supplement their go study to consider giving the audio go lessons site a try. There are a number of free lectures there which you can view before you make a committment. They have one called the 2007 Chinese New Year Lecture which is about handicap play.