Bad grammar drives me crazy. I can see my family bracing for a diatribe now. :-) Grammar is very important. It helps to ensure that what I'm trying to write is what you're reading. Little things make a big difference sometimes. Some grammar rules get a little too fussy though, and can obscure more than they clarify, like when you try to rewrite a sentence so that it doesn't end in a dangling preposition. "For which it was intended" is just more awkward than "It was intended for," grammar be damned. That's all I'll say about that though, mainly because I'm afraid that someone will come along and say, "It's funny you should say that, because I've noticed that you consistently make this grammatical error...."
My spoken grammar is a whole different story. Can I just say that I'm a Southerner and leave it at that? I might know better, but that doesn't stop me from pulling out, "I ain't got no" on occasion.
I own a Chicago Manual of Style, but I don't actually pull it out too frequently. It's too easy to just Google whatever I'm not sure about. I've read Eats, Shoots and Leaves, but thought it was pretty basic and I didn't really find the snarky humor all that funny.
How do you feel about grammar?