When I worked for an online travel agency a decade ago (now just a brand for you-know-who) we got lots of nasty emails about Spirit Airlines, as they were always the cheapest option in our flight booking, but after you booked you found out all the extra charges they required, none of which were given to us to add in the total price. So the price we showed was not the price you would likely ultimately pay (we always joked internally that they charged for air to breathe). But we got all the grief. Also, they had their own flight reservation system, and it was dog slow (at the time it could take more than a minute to make the actual booking).