I strive to use good grammar. Like sammi-san, I'm a writer, so grammar has been something that interested me all my life. The rules stuck in my head, so despite the fact that I don't care as much as I used to, I can't fully let go of "proper writing." It's stuck with me so well that I'm known for my grammar knowledge in real life as well as online.
I used to judge others by how they wrote online, but then I grew up and decided not to care. It really depends on the setting, though. Like here on PC in a casual post, if someone uses bad grammar, I'm not going to hate them. But if it's a story posted in my section, I tend to notice it more because a story should have proper grammar. Or if a reviewer is telling someone to improve their "grammer," then it's funny. Otherwise, I'm more lax towards others using proper grammar. Especially when speaking. When I'm talking to someone, all rules are broken.
Really, grammar should only be judged when it really counts. If someone is applying for a job or writing a paper for school, then grammar should matter. That's professional. If someone is in a casual setting, then grammar shouldn't matter because that's just silly.
Though apparently I give off a vibe that I care, because when people are around me, their grammar improves. I just use good grammar because it's what I'm used to. Everyone else is free to just be readable, since unreadable text is just a big no-no.