Pros: Decent service. Decent on-time record. Boeing 737s everywhere.
Cons: No frills. No assigned seating.
I don't travel enough to have a "favorite" airline. And, since Charleston, SC (CHS) only offers a few carrier choices anyway, I ended up choosing Southwest for my recent trip to Las Vegas (LAS).
PROS: The website was simple and easy to navigate, my ticket cost was fair (well, maybe not really "fair", but comparable to other carriers), the planes were reasonably clean, the attendants were mostly personable, the no assigned seats game was "fun", and all legs of the trip arrived/departed on-time. And, on top of that, SW only uses 737s, so you know that you won't have to ride any "puddle jumpers"!
CONS: These flights are truly "no-frills". If you want food or entertainment, a guaranteed seating location, or even a more-than-the-minimum sized seat, this is not the airline for you. WiFi access was only available on 1 out of the 4 planes I rode (for a $5 access fee!), and a small pack of crackers or nuts is the most I was ever offered (even on the 4-6hr flight legs).
Overall, I had a very good first experience and was very satisfied with all aspects of SW's service. Don't let the "no reserved" seating issue scare you. There is really nothing to the system SW uses; and, even in a worst case scenario, the seat you could end up in will not likely be a whole lot worse than any other on the plane (Sure, those SW 737 seats could be just a little wider. But, at least everyone is sitting in the same sized seat!).
I'll Fly SW again!