I tried to learn Spanish a couple years ago with one of those CD language learning programs, but when I actually tried a conversational program with a teacher, it was amazingly easier. I’ve never heard of anyone trying a CD program to learn English as a second language, but with all the nuances, expressions, strange pronunciation and spelling in the English language, which all of us take for granted, I would not want to try that without a teacher to guide me.
The CD language learning programs will, to some extent, teach you language aspects and you will be able to use some of them on your own. They are cheaper and you can do them on your own time (even though you’ll probably do them in the car when you’re also paying attention to driving, so you won’t even be focusing on the lesson!). But what if you have a question? Who will you ask? That is the main reason why I prefer conversational ESL classes: language is meant to use with other people, so I want to learn it from another person and practice it with other people!
Teachers in a conversational English class are trained to let the students speak as much as possible and then correct them when they make mistakes. I really don’t see a downside to this method when you will learn the language correctly, faster, and have a better grasp of what you are learning. If I needed to learn any language fast or slowly, I would undoubtedly choose a conversational class over a CD program any day.