It's great that Potbelly's is starting to pop up around the valley. I was a big fan of them before moving here from the Midwest.
Since they're a national chain, the food is (probably) consistent wherever you go, but my favorites on their menu are the turkey & swiss sandwich (with lettuce, tomato, and italian seasoning) and their milkshakes, which come with tiny cookies on top--the oreo shake even comes with tiny oreos (!) on the lid.
The staff was super friendly and the restaurant was clean. I'm not usually downtown--we were just grabbing a quick bite to eat before a concert at Symphony Hall nearby--but I'd hit up this Potbelly's again if I'm in the neighborhood.