We have always flown with Allegiant Air to Florida because they are the only airline that offers a direct flight from where we live. I do not want to deal with delays and missing connecting flights on vacation. When we went for 11 days, we paid for 1 suitcase; but last year I took my DS4 for 7 days and we each took just a carry on and backpack. You do have to pay for specific seat assignments, but we've never done that. Because we have a young child, they let us board after all the paid assignments and we've always been able to sit together. My DH and I went by ourselves once and we found seats together. We've never had unfriendly service with them. You also pay for any drinks or snacks they provide. This is not a big deal for us, as I just pack our own supply of snacks & we fill our water bottles after we get past security. Because there are 5 of us, we just need a basic safe flight for as low cost as possible.
Where we live they turn their flights around pretty fast. A half an hour before your flight leaves, the plane comes in with the flight from Florida. They come off and you board about 5 minutes later. We have left late a few times because of this tight schedule, but for us it is worth it since we have no other direct flight options.
Allegiant Air is not for everyone, but for this time and place in our lives it is the best option.