Dunkin' Donut's has pretty good coffee consistently - but there's another element that's in play driving its popularity here in Maine (well most of New England): class consciousness. Dunkin' Donuts has grown to clearly and explicitly represent an unpretentious, blue-collar ethic, as opposed of course to the "premium" coffee shops. Holding a Dunkin' Donuts coffee is shorthand for "I'm not a snob." Hence their "America Runs On Dunkin'" ad campaign, which clearly and deliberately links Dunkin' Donuts coffee with work -- difficult, stressful work at that.
Class issues like this are powerful in New England, which has a heritage and a national image that orients to Boston old money and snooty Cambridge liberalism, but which in actual fact has a higher-than-average population of union members and manual laborers, and a long tradition of mixing social class with its politics. (Hence the endless fascination with the Kennedys, who for the enormous Irish population here represented a dramatic breaking of the walls that had kept the wealthy apart from the general population.) The recently elected Sen. Scott Brown, with his pickup truck and "people's seat" rhetoric, reflects the same ethic, if from an unexpected partisan direction -- and he was a Dunkin' Donuts man if ever there was one. (True story: Scott Brown's first job was in the kitchen at a Dunkin' Donuts in Wakefield, Mass.)
The disproportionately intense fandom for the Red Sox and Patriots, I'd argue, demonstrates the same thing: sports fans here use their fandom as a socioeconomic identifier as much as a regional heritage identifier.
I've traveled through Boston (minutes, in fact, from Scott Brown's hometown of Wrentham), many times and Dunkin' Donuts is ubiquitous. There are three within a five-minute drive of the convention center, where DD and Starbucks coexist -- the class difference in their clienteles is dramatic and obvious. Central Square in Cambridge tends to host crowds of construction workers, painters, and other blue-collar types waiting for the bus alongside streams of office workers and Harvard/MIT kids; almost to a man, you can identify them by their coffee choice. I'm not suggesting that class is the primary motivator of DD purchasing, of course, but it is clearly an element.