As is our habit, my wife and I chose this airline based on past experience and value. In fact, even though we live close to O'Hare, we are willing to travel all the way to Midway, on the south side, in order to fly Southwest.
Two checked bags are free and there are (as advertised) no hidden fees.
Since we purchased the Early Bird package we were automatically checked in 36 hours prior to our departure. This allowed us to print our boarding passes at the airport on the way back instead of trying to find a printer at our hotel.
Boarding is done in groups of A, B, and C. and the Early Bird check in pretty much guarantees boarding with the A group. This is important in that there are no assigned seats and you choose your seats as you board. By boarding in the A group, you're able make a selection that works for you.
All of the staff are pleasant and professional and, in the case of the flight attendants and pilots quite entertaining.
With all of the possible headaches with other airlines, Southwest is a refreshing change.