Experience shows that due to the special ways that the human mind works, no product – no matter how advanced – will ever be more effective than working face-to-face with an instructor. A product can be considered successful if it is at least half as effective as in-person instruction. This is due to the increased independence that one has when working alone with instructional materials. Instead of closely following the instructions, novices will stubbornly do whatever seems right to them. They will nevertheless send all their complaints to the author. However, it’s nearly impossible to remotely instruct people in how to analyze their mistakes, let alone track their progress.
Financially speaking, creating instructional materials can bring in a good and steady stream of income, especially if you build your brand and keep your work from being pirated. Once you have this kind of business set up, you might consider yourself to have "passive income," i.e. someone who enjoys a steady income from an operation that requires little personal participation.