Yup, came here to say this. Once you're on the plane and its in the air, Spirit and Frontier are like pretty much every other domestic airline. There's slight variation in terms of whether you get a whole can of coke for free or not. If you're taller than me, the 28" of seat pitch vs say 31" on delta may make a difference, but I'm only 5'9".

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.