Here's my experience. I used to fly Delta almost exclusively. Loved Song when they were available. BUT...I got tired of having my flights constantly change. If I booked more than 3 months out, my flights would change mulitple times. And many times my flight would be cancelled and I would be put on a flight that was almost full....so lost my seat choices. Drove me nuts. Then, I had to pay for each checked bag.
But, I was really afraid of flying with SW. So, I started flying with Jetblue. And I really like them. But, SW started having some really good fares..almost half what JB was charging for just about the same times. So....I took a deep breath and booked with SW. I've never looked back.
I love SW. No, you don't get an assigned seat. BUT with the new boarding proceedures (maybe not so new now), you don't have to stand in one of three cattle chutes, never leaving you spot, for up to two hours, in order to hold your 'spot'. Now, you get a boarding number, so you can sit and relax. When boarding is called, get up, take your spot and board. So easy.
And not having to pay for my checked bags is wonderful. All in all...I love SW. Since they don't open their booking window as soon as the legacy airlines, they don't make changes. Sure, there could be an equipment issue close to departure time, but there's nothing you can do about that. Generally speaking, when you book with SW, you pretty much can count on that flight being the same as when you booked it.
No way will I fly with Delta unless I absolutely have to anymore. For me, from Boston, it's Jetblue. From Providence (TFGreen), it's SW. I switch between the two pretty consistently.