I stopped at Dunkin Donuts for an apple crumb donut this morning. I think their coffee is MUCH better than Starbucks. And if money is an issue for you, it's half the price.
To me, it's a no-brainer. If you sell doughnuts (as your main item) you MUST have good coffee.
I'm not much of a coffee drinker anymore, but I love DD's coffee.