I booked basic economy last month. They let me check my "heavy carryon bag" at the gate for free on the outbound trip. Seat assignment was made prior to my arrival (same day) and not at the ticket counter. However, the counter agents were happy to switch with me for free if economy seats were available. And, once the airplane door closed > all empty seats were fair game except Delta comfort or first class. I think its a great deal if you can do it all with a carry on.