I JS'd to Madrid about a year ago on DAL out of ATL. Show up at the International service counter at least 90 minutes prior to go time and get passport listed.
Worked great.
Coming back, get to the airport at least 2 hours prior so you have plenty of time to pay the departure/extortion tax. Also, if you take DAL, make sure that you know exactly how your comapany is listed in their computer to verify that you can JS on them. Not much of a big deal hear, over there it can be with the language barrier.