What is your opinion of Southwest Airlines?

we are planning a cruise that leaves from Ft Lauderdale FL. In the past we always flew Delta but a year ago our plane was over booked and we got left at the gate in Atlanta and missed our sailing---WILL ONLY USE DELTA IF WE HAVE TO!!! We are considering flying Southwest but have heard that the seating is first come first serve. Is this true? If so what can we do so that the two of us can be able to sit together. Any extra advise from frequent Southwest fliers would greatly be appreciated.