Cheaper and the coffee doesn't suck. The Starbucks coffee is too strong/bitter for me, generally. Coffee rules here in New England, not Starbucks.

I prefer Starbucks coffee over DD, but I put a lot of milk in my coffee. I don't know that I could drink Starbucks black.
DD has better iced coffee, though. I really like their iced coffee.

Real coffee shop > Starbucks > Dunkin Donuts > everywhere else

I like extremely strong coffee, especially espresso. I very rarely go to any of these places anymore though since I got a Nespresso.