Here is an interesting article which shows that grammar correction is not very effective in improving language skills.My own experience does not entirely agree with this article. I have found that some feedback on my writing and speaking can be helpful. It is not so much the explanations, but the constant reminders of my mistakes, which gradually help me to notice things in my listening and reading. It also seems to me that the acts of writing and speaking are in themselves beneficial to learning,with or without correction. But for improvement to take place,the learner has to start noticing what is happening in the language. In this regard, correction as a form of feedback can be useful, as long as there is no expectation that correction will lead to mistakes disappearing as an immediate result of the correction.