I don't mind when people say "OMG" when they chat with me, but if it's used repetitively, it does get a bit annoying. I think it's fine when it's used once in a while. A couple of my friends do use it a lot, usually when they're surprised by something. When they say it too often though, I warn them about it. I mean, how often can someone be surprised right? You don't want to overdo this expression.