We haven't flown Allegiant - but did consider it while planning for our WDW trip in March. The reasons we decided not to are: A) it goes to the Orlando/Sanford airport, which isn't bad in general, but you can't use Magical Express then, or if you rent a car, you have to add in extra driving time. B) The rates ended up being more than flying American from Lambert. C) Since Mid-America has had airlines come and go so quickly, we wanted to make sure that our flight was going to happen for sure. D) Hubby's company wouldn't allow him to fly Allegiant - it had to be one of the major airlines leaving Lambert in order to be paid by the company for the conference he is attending in Orlando (while the rest of us are at the parks). Since it would be the first time our kids fly, I didn't want to have them on the flight alone, so we are all flying American.
With that said, I wouldn't mind trying them out next time if the fares are more competitive and we had a little more flexibility.
Sorry I couldn't help more!