I teach grammar every day. I have two-year-old twins stumbling their way into the English language, and a dyslexic Montanan husband who swears that in the West, that's just how people talk: "had went" is as good as "had gone."
I'm also an editor and a journalism professor, and I spend a fair bit of time trying to puzzle out how to teach college students to write. The question that devils my teaching colleagues and me is this: If you don't know grammar by the time you're 18, can you ever learn it well enough to be a professional writer? Beats the heck out of me.
At my college, students are required to take a remedial grammar course--that's a lousy word, remedial, but it's the truest I can come up with--and for many, it's a painful experience. In the worst cases, students don't want to take the class in the first place, they have grammar stage fright once they're in class and they come out of the class having memorized just enough to pass some quizzes and a final. End of grammar acquisition.
Gerald Grow, one of the leaders in journalism/mass communications higher ed, wrote the best academic piece I've come across, where he wades into sensitive issues, regarding the intersections of language, race, culture and personal motivation. Here's a link to the article: http://www.longleaf.net/ggrow/
And while we're on the topic of unsatisfied college students, here's my favorite RateMyProfessor review:
I couldnt stand this woman. She thinks way too highly of herself, and she was not funny! I didnt learn anything new i didnt know from 3rd grade phonix class. I would not reccommend her. (grammar student, 2005)