I don't jump down people's throats about it. It doesn't particularly annoy me, but it's really not difficult to learn how to use English correctly when it's your first language!
To be fair - I shouldn't really shout as my grammar in French is completely fucked, but in English I flinch whenever I read something I've written something and spot a horrible grammar mistake in it.
There is however one mistake which is just plain rubbish: When people say "should of" instead of "should have", "would of" instead of "would have". It actually doesn't make sense. At all.
Choose any phrase or sentence with either the word "of" or "have" in it, and replace the word with the other. Does it make sense?
"Lord have the rings"
"Of a nice day"
"Where of you been lately?"
"What of you been doing?"
"Have course I'll do that"
etc.