I really have to agree with DK on this one; bad grammar oft times ruins a good story. Personally, I use MS Word to write all my stories in, and sometimes the grammar check on it doesn't agree with me. However, it has caught a lot of things that I've gotten into the habit of doing.
Grammar and sentence structure is very important to get correct, but, at the same time, in character dialogue... it's more believable if some of your characters get it wrong.
In the same vein, I just finished reading the book "Exile's Honor", by Mercedes Lackey (exceptional author). The main character, Alberich, is from a country whose language structure is... at the expense of sounding hypocritical... bass ackwards (and they'd say the exact same thing about English). Hence, during most of the story, Alberich comes across as sounding like Yoda when he's speaking in 'Valdemaran'; because, to him, that's how the sentence is supposed to be.
But, I digress... bad grammar is just bad