Ian Gilfillan has an excellent post on McDonalds as a corporation, and how we view it. He actually sums up my thoughts and experiences fairly well - the cognitive dissonance involved when you try to tell people how bad McDonalds is is unbelievable. I have spoken to people so many times who say "Man, I had McDonalds for lunch, and now I feel absolutely awful". And I've heard them say it so many times! But try to point out that that's completely unsurprising, and that McDonalds really, really is absolutely awful for you, and they just say "Yeah, but it tastes nice", and refuse to engage further.
People are a bit stupid, I think.