I had never flown Allegiant Air and was told that it was a cattle car experience; I found it to be quite the opposite.
There are the well known streamlining of costs of course. It is a $20USD fee for every piece of luggage that is checked; maximum weight is 50 lbs (22.5kg) and they are strict on charging for overweight bags. Therefore most people take a carry on and a personal item, such as a purse or a lap top. The carry on items must fit into the overhead bins or under the seat. Snacks and beverages are charged for.
The seating however is interesting. Those who have paid the extra for pre-assigned seating are allowed to board the plane first. This is followed by seating those who need assistance, and then families, with small children under the age of 5. For the remainder of passengers, they will find on their boarding pass a "Z number". Because open seating is on a first come first serve basis, passengers want to check in as early as possible. They call Z 1 -10 and those people have an opportunity to enter the plane and choose the seats they want. In other words Z stands not for seat rows, but seating sequence. Those who have the highest Z numbers will find themselves without much choice for a seat.
The flight was smooth and the staff professional.