It's fast food chain done right -
high quality - they only locate stores that they can reach from their own distribution centers, always fresh food
consistent experience - these places will always be clean, they will have cheerful staff, you know what's on the menu and there aren't any surprises.
good prices - a hamburger, fries and a soda for under $6 - with taxes - how often can you say that?
the menu is simple in terms of the basics of what is offered but highly creative in terms of how you can get it served (animal style fries - who else does that?)
it's an experience, it's the kind of place you probably won't go to every day but when you do, you know that you'll get a great burger at a good enough price that you don't have to skip the fries.