I have a couple friends who fly them, mostly on account of their all-you-can-fly $250 pass [1].
[1] https://www.flyfrontier.com/deals/gowild-pass/?mobile=true
[1] https://www.flyfrontier.com/deals/gowild-pass/?mobile=true
But, their website has always given me tons of problems. My ability to successfully purchase a ticket often felt totally random.