Southwest is one of my favorite domestic airlines to fly with. Their prices are reasonable and there aren't any surprise fees (i.e. paying to check in your bags or for drinks & snacks on the plane, unless you want an alcoholic beverage). Nearly all of the staff I've encountered were very friendly, which is always a huge plus in my book.
My only complaint, particularly at the Southwest location in the Phoenix airport, is the lack of seating at the gates (where you wait for your flights). Every time I have a lay over here that tends to bug me since I'm not trying to sit on the floor. Other than that, I'm a fan and will continue to fly Southwest.