This is 100% spot on. Those saying Allegiant isn't any different than the major carriers like United, American, and Delta must not fly very much. I do 60-100 flights per year and its a huge differentce. Allegiant has minimal to no flexibility, small seats, no frills, greatly reduced flight schedule, can't swap planes out when something goes wrong, and their customer service will not help you out if they do cancel a flight.
When everything goes smoothly, Allegiant is fine. But if you get caught in delays or cancellations, it really, really sucks. But its cheap and flies direct to a lot of places and it has its place. Just be aware of the pitfalls with them.