This is an odd one - I'm not keen at all on McDonalds, however during university I loved KFC and Burger King. Whenever we go to the USA my wife and I love Wendy's.
But they're all basically the same - fast food, burgers, fries etc. In my head I refer to them as "glorified cafeterias" - you go, order your food, pay, get it on a tray and then try to find a table. Eat, bin the remains.
In some cases it is a treat - we love Wendy's, as I said, because of the good combination of salads and burgers, and the frosties they do. I like KFC and Burger King because they tend to do larger and tastier (to me, at least) burgers than McDonalds.
But in regards to eating, if given the choice we'd much rather go to a pub/restaurant place, or get a chinese. You tend to get much more for your money and they're tastier (and probably healthier), as well as a larger choice.
But that's besides the point - why do so many people think they're a treat? Because they're convenient and fun. I know on many holidays as a kid I'd always want to go to McDonalds (and it's ilk), and not one of these boring proper restaurants. Nope, give me somewhere that caters to me, and I want sodding clowns on the wall and a toy, and a burger covered in tomato sauce! As a kid, that is (not now, that'd scare many an adult). As such, I think many adults have fond memories of McDonalds (and it's ilk) doing warm, substantial and convenient food - it's what they're used to.