I used to fly Frontier all the time. In the last few years things have changed and not for the better.
I don't mind the add on fees but they often cancel flights and given their small fleet, this can mean days of being stuck. They had a mass walk out last December and thousands of people including my boss we're left without a way to get to their destinations right at Christmas. Frontier claimed it was weather related but in fact it was a walk out. Baggage sitting around airports, people stranded. Total mess.
So be prepared for anything. The planes have been configured with as little leg room as possible. Seats are thin with virtually no padding, they don't recline and the arm rest is about six inches.
And if you fly them get to your gate very early. They overbook like everyone else but they have a lot of cancelled flights and since they are so overbooked they board early and you can show up on time to a plane that is full and closed. This happened to us last September and we had reserved seats we almost lost in spite of getting to the gate on time early.
It makes me sad because for many years my family flew Frontier ten plus times per year. They treated their customers so well. But then they changed hands and it's been down hill.
Many, if not most people will fly with no problems. But if you encounter a problem it will sadly be one that could cost you a lot of time and money.
And that walk out in December was a result of pilots who felt cost cutting had made things unsafe. Which is why I will never fly them again