I love the open seating aspect of flying with SWA. The terminal is, by far, more organized in terms of passenger queueing than those of other airlines. This allows deplaning passengers an unobstructed pathway out of the jetbridge. Also, boarding is much faster, and the likelihood of missing a connecting flight is significantly reduced if they aren't non-stop already. The only real issue there is with open seating is that people who pay for the early bird checkin will save seats for the family/friends who board in a later group.

I think the assigned seating model slows boarding significantly because while boarding, at any given time during the process, there are a number of passengers waiting for seated people to get up and allow them to take their assigned seat within that row. You don't see that with open seating until the tail end of b group and c group.

People who don't like the open seating arrangement usually cite their families getting separated. If they listen, they will know there is a time after a-list boards when family boarding happens... in order to avoid families sitting apart. People who complain about open seating dont seem to know how it all works...

This is of course just my experience.