There a many avantages of learning the language by yourself but in the same time there are many disadvantages. You can learn the grammar, many new words, expressions, but without a teacher it is very difficult to learn how to use them in the right way and of course how to say. It is kinda difficult to gain a good pronunciation by yourself. Learning a language is of course hard work, much reading and dedication. What I reckon is that to learn a language well is needed to learn by yourself and also with help of the teacher, due to the combination of those two you can easily achieve a proper level of knowledge and get to the top of the ladder.