Since I always have checked baggage I've not used the online check-in. However, Delta offers check-in kiosks at the airport that are quick and fantastic. You will need either your Delta FF ID card or the credit card you used to purchase tickets. Just slide the card through like an ATM and your itinerary will pop up. You indicate the number of bags you are checking in, answer a couple other questions and the machine spits out your boarding pass. An attendant will tag the bags and off you go.
The first time I used the kiosk a couple years ago (at Logan) I thought something was wrong when I was all checked in and on my way in a flash while others were waiting in the usual long check-in line.