Personally, I try to make sure mine is correct. I tend to slip up, though. My biggest flaw is remembering when it's supposed to be who and when it's whom. D:
As for other people, if it's in general posts on the Internet, I really don't care. Not enough to correct them, anyway. I might cringe a little on the inside, but I'm not going to correct them. In a more formal setting, like writing or articles or office settings, I hate seeing them. But I'm still not correcting them. It's not my place.