Their dollar menu is pure genius.
I stop and get a $1 coffee (most of the time decaf) every morning on the way to work, as do a lot of people I know.
Their coffee is actually pretty good- better than most gas station fare.
When you order, it isn't much to throw in a $1 Sausage McMuffin or two- it ain't easy to resist.
The stores that run efficiently, tend to draw a lot of customers, regular customers.