Really enjoy Southwest Airlines - some planes are nicer than others and you HAVE to remember to check in on time to get a good seat. If you forget to check in you will have a seat in the very back. Other than that great airline.
Feel like it depends what plane you get - I have been on really nice spirit planes and some not so nice ones. Also may look cheaper but once you add your luggage, seat, and buy a drink on the plane it may have been cheaper to go with different airline.