We have flown Allegiant several times over the past few years and mostly have not had any issues. Our one time that we had an issue I can honestly say that Customer Service is almost impossible to get hold of and are not very helpfull when you do finally manage to get a hold of an actual person.
They are an Ultra Low Cost Airline which basically means EVERYTHING is an additional cost.
Baggage fees for Checked bags as well as Carry-on luggage.
Seat selection $12-25/seat depending where you want to sit on the plane. If you don't care where you sit and don't care about sitting together, you can opt not to purchase.
On board drinks and snacks are available for a fee.
It should also be noted that they fly into SFB, which while technically is in Orlando, it is not serviced by DME and is a 45-60 minute drive to WDW.
A rental car is definitely your best option when flying into SFB. You will need to factor this into your
budget.
SFB is a nice small regional airport and really easy to get in/out of, an absolute delight compared to MCO!
Allegiant is flying an aging fleet of mostly MD-83's and MD-88's with a few A319's and A320's in the mix (mostly for the longer haul routes)
They have been in the news quite a bit lately for mechanical issues with their aircraft, pilot issues and flight attendant issues.
Check the following link, there are already 8 delays/issues today and there were 14 delays and/or issues yesterday:
http://www.allegiantair.com/travel-alerts
Allegiant typically only flies 1 or 2 days per week from your local airport so a mechanical delay can cost you several hours or even days of your vacation while the plane is repaired or a new one is flown in for you (Allegiant does not have agreements with other airlines to handle issues like this).