I jumped ship 3 years ago. No regrets whatsoever. Spirit is a good airline to work for. But I'm much happier where I am now. Everybody is different though. How many years do you have left? Do you like international flying?
No more 4-6 month upgrade but upgrade still at about 3 years on the small jet or fly right seat on a big jet.
I left NK before the new contract had been negotiated and it was very refreshing to work for a company where the atmosphere in the cockpit was always positive. Hopefully that has changed for the better at Spirit.