My experience
Well, my experience doesn't come close to what the previous person dealt with, but I, too, will avoid Frontier if at all possible.
I booked Frontier from Denver to LA, and then Delta on to Honolulu. Then I began reading all the negative comments about Frontier, but hoped for the best. At first, everything was fine...staff helpful, my seat OK. I had checked in at a kiosk, even had some help from a Frontier person; twice, while checking in, I read on the kiosk screen that my luggage was checked through to Honolulu. Since that was what I expected, I had no reason to question it. Got to LA, had to take the shuttle to the terminal serving Delta, got through TSA and I am at my Delta gate when my phone rings. It was Frontier saying I had to go back to the Frontier terminal and collect my luggage because they did not have an agreement to send it on to Honolulu.
Nothing I said made any difference. As I was alone, I had to lug all my carry on stuff with me back to Frontier, get my large suitcase, and trudge back to Delta and do the check-in and TSA bit again. I was not a happy camper.
I told Frontier I would tell my story as often as I could and announce each time "I will never fly Frontier again!"