It's quite a widespread question on the Internet. And, of course, there are different points of view. Some people prefer to study language by themselves explaining this with the following reasons:
1) The prefer to study at their own pace - people just don't want to wait for someone or keep up with super fast students; 2) Some people are very shy and they are just afraid of asking any questions in front of other students or participating in discussions;
3) Some people just don't want to spend a lot of time doing some exercises during the lesson as they can do it at home without any help (the point is: "I can do it at home and if I  have any questions I'll ask them later" )
These statements are pretty convincing but there is another "camp". Some people say that the only right way of studying is studying with a teacher. And they explain it with these arguments:
1) If you don't want to make mistakes and repeat them over and over again you definitely should study with a teacher - he/she is the only way to avoid them;
2) teacher (classes) can provide useful practice - you can work in groups and have conversation on different topics. Live contact is a very important thing;
3) Teachers usually give some extra information about every topic and also can explain new ones using the most appropriate and simple examples.
As for me, I believe that the truth is amid these two ways. The most effective studying lies in both attending some language courses and studying by yourself. Of course there are some more factors like - is the teacher good or not, are you a lazy pig or not)