While I'm not a fan of UA over the last few years, it is still a nicer airline to fly than Spirit. I just flew Frontier for the first time last weekend and it was pleasant for TTN-MDW. But, unfortunately, it looks like Frontier will be turning into Spirit 2.0. Canceled flights, being stranded, horrendous pitch, out of control rude customers--while I know people say it all the time--I won't fly Spirit ever again. There actually have been times where Spirit had good fares to Florida, but I chose not to go on a trip.
I hope Spirit doesn't bring every carrier down to their level.