go with the specials and brunch. everything i've tried from foster's has been at least above average, and most is excellent. it is very pricey, though. most sandwiches run about $8 and you don't get anything on the side at that price. in addition to the standard menu, they usually have "local special" sandwiches with ingredients coming only from local sources which are sometimes worth the cost. their standard menu sandwiches and sides are very good, but not worth what they charge imo.
their brunch is a different story. it's worth every penny of the $8-9 the average plate costs. you get a choice of pancakes/french toast, omelette, and a chef's special, which usually consists of an herb biscuit, very flavorful grits (herb, tomato and goat cheese, etc.) or polenta, fruit, and another meat (italian sausage, lump crab meat, salmon, etc.). their coffee is a custom roast from counter culture and also recommended. during brunch in spring and fall, foster's is typically packed with duke students. prepare for a reasonable wait and be ready to hunt for a table inside if it's cold out, but it's worth it.