I've flown with allegiant many time, and you get what you pay for.
The good: The fact that you can fly to many places cheaper than you can drive there with them makes up for all the headache you deal with. Since I would rather sit in an airport/on a plane for 3 hours than drive for 6h. They have an online checkin so no paper work, fast and easy.
The bad: the customer service is okay. The workers are contracted out, so they have little knowledge about the policy's, and more over, little power to do anything about it. When you need to contact someone ie) my flight was delayed and no one bothered to tell me about it like they were suppose to online ie) they lost my bad and didn't call me when the bad was in so I had to drive to the airport daily to find out the status.
The ugly: the cancellation policy cost more than my flight most of the time, so it's cheaper to rebook a flight than reschedule. And the other passengers have no consideration of each other. The fellow passengers truly are the worst I've seen on any airline: talking on speaker phone, reeking of alcohol and fried food, picking their toes, leaning all over your tiny seat.