I am booked on Frontier for Star Wars in April. It wasn't just the cheap price for me, it was also the only nonstop flight to Orlando from Little Rock. I usually spend $400 plus on a round trip ticket to Orlando and it is 6ish hours with layovers. I can suck it up and sit wherever they seat me for a 2 hour nonstop flight of $80! I will bring my own drink on and snacks although I think I can live 2 hours without eating, lol. I paid the same for checking my bag with them as I do with Delta so I didn't mind that. I would probably pay for a carry on if I was running a marathon but I will just wear my running shoes and if anything happens to my bag I will wear some of DD29's running clothes for a half marathon. So I agree with
@huskies90 that if you don't care about all the extras they can be a good deal.
ETA: I have flown them before (to Denver) so I know what to expect and not expect!