Love the fact that I can get non-stop flights to Milwaukee with Southwest from several locations. This flight took off on time thanks to the quick and easy way boarding is done on Southwest flights (and arrived ahead of schedule). You take your assigned place in line prior to boarding, and then pick any available seat once you're on the plane.
I also find their seats seem to be a bit wider, with enough leg room for a 6'1" person, than other airlines. Not to mention you can check 2 bags at no extra charge. Even if you only check one bag, their ticket prices become cheaper than other airlines' most of the time.
The flight attendants are often quite funny while going over the safety features of the plane, which puts you at ease.
I always try to fly Southwest, if possible.