> Once you're on the plane and its in the air, Spirit and Frontier are like pretty much every other domestic airline.
Yes, if you ignore the part where things are different, it's basically the same. Trouble is, those differences do meaningfully make a difference. There's no objective measure for misery and happiness, but flying Jsx is nicer than Spirit. You can put a dollar value on misery, that's why one's so much more expensive than the other.
I still avoided them like the plague because the legacy carriers are selling you operational performance and the ability to usually get you where you're going within a reasonable timeframe if you're delayed or canceled. Spirit, Frontier, Allegiant, whoever else, do not do nearly as good a job when something goes wrong. Although they should get a lot of credit - none of them have ever had a fatal crash.