When it's all said and done, Allegiant is not the cheaper way to go. By the time you pay all of your bag fees, seat fees (if you want to sit together, I suggest you pre-purchase your seat assignment on this airline) AND the extra $250.00 (or more) for RT transportation from Sanford to WDW, it's actually more expensive. Our "cheap" airfare came to $700 when it was all said and done with the extra fees. Not to mention the poor customer service and delays (we also had those happen to us). Adding the $250 for extra transportation, we wound up paying almost $1,000 just in transportation for two of us.
After this most recent experience (December 2012), I will fly into MCO from now on.
Check out Allegiant's Facebook page before you decide.