Since its a 3 week stay adding up ones costs for the extra days youd be there to get this cheap fare can far outweigh any savings. If a person has a place to stay for free and wont be lossing any $$ from taking off from work then its not bad at all. But if youd be renting a car or staying at a Hotel or renting an apt by the day or week, it will most Likely be alot cheaper to pay more for your tkt and fly later and return earlier.
BTW DL wants $1211 all in so $999 isnt really that great of a savings on the non-stops. 9/13 - 10/7, leave 9/12 and its $1145