I think it's a great way to expand your knowledge of a language, but I personally wouldn't rely on it alone. I would definitely take formal classes as well, simply because that's the way I learn best, by having a structured approach and the ability to ask even the most complex questions concerning grammar that most native speakers simply don't know becuase it's natural to them. Of course, there are people who find this approach terrible for them because of how formal it is, and I think language sharing is a great alternative, as well as a great supplement to formal teaching where a student is able to practice in a real world situation what they've learned.