We have flown with Allegiant once. It was a total pain, and I would only do it again if it was a BIG savings. We went to the airport to book to save the $15 each for 5 people. We were told by their reservation # that the ticket desk at the airport is only open for an hour before and an hour after arriving or departing flights. When we arrived an hour before a departing flight, we were told that they could not help us because they were too busy checking in departing passengers, so we had to wait an hour until they had checked everyone in. They still didn't want to do it, because the ticket agent was also the gate agent, and the 5 people sitting around chit-chatting behind the desk obviously had some critical duty that was invisible to customers. But I pushed since I was there with my mother in a wheelchair and a 3 year old and didn't want to have to make the trip again!
They nickel and dime you for EVERYTHING. Extra charge for seat assignment, extra charge for preboarding, no food or beverages served inflight unless you buy them. Also, you cannot print out boarding passes online unless you pay for the seat assignment.
I personally thought the airline was a joke. But it got me from point a to point B. The whole process was extremely unorganized and completely lacking in customer service, so you get what you pay for in this case.