I worked there as a student and it will come as no surprise that is when I became a vegetarian. In fact I wasn't the only one, other colleagues (including management) also became vegetarians as a result of working at McDonalds. That alone speaks volumes.
As a company they treat their staff well and generally I enjoyed working there, but the food is not suitable for human consupmtion.
Jamie Oliver recently exposed that the red meat used in beef patties is destined for dogs; cainines who have the stomach for the meat used, humans do not and yet it's going into their patties. Anyone who eats at a fast food resturant is taking their life in their hands IMHO even before you take on board animal welfare.
I go in there rarely and even then it's usually with someone else who wants to go in not me and even still I have little more than fries and a bottle of water.