We just flew Allegiant and if you book it right there are no add-ons, you just select the package you want. They are typically way cheaper between the city pairs they service and all of their flights are non-stops, no connections. It's also not as though other carriers don't charge for checked bags and seat selection as well as actually not allowing carry-ons at all at some fare tiers.Looks like Allegiant Air has best pricing on Kayak. Allegiant Air pricing also has add-on fees for nearly everything, so you are not close to your real final cost.
In our case, flying CVG to JAX and return Allegiant was a couple of hundred dollars cheaper than were the majors, including checked bags, carry-ons and seat selection. The outbound A320 and the inbound A319 were both clean and the flights were on time.
Having never flown the airline before, I'd fly them again and would recommend them.