Allegiant is not a bad airline for quick and cheap travel, I have been using them to fly into Bellingham from Vegas and the price is always half of what Alaska charges even with the carry on charge.
It seems like most complaints are coming from people who try to change flights, they make it very obvious that if you do not opt in for their travel insurance you will pay $75 for a change fee along with the difference in price. Seats are not comfortable, I usually pay the extra $20 for exit row or front row which give a tall guy like myself a decent amount of leg room. They do charge for snacks and drinks, just come prepared pack some snacks in your carry on and bring a empty bottle of water to fill up at the airport.
If you pay attention and understand their policies and realize the leg room will be less than a normal airline Allegiant is worth it. I am going to deduct a star due to their less than stellar safety record and their planes are sometimes pretty dirty inside.