I flew Allegiant once between Fort Wayne (FWA) and Orlando/Sanford (SFB) in early 2011 for a trip to Universal and Kennedy Space Center. The flight was fine (no delays either way) and the vacation package deal was unbeatable, even with the extra fees added. I actually would have paid more to get a similar deal out of Indianapolis (IND) on Southwest (and that's not counting the extra gas and higher parking fees at IND).
One thing about Allegiant is that they encourage booking your flight and hotel/car together through hefty discounting of packages. They do offer 6 Disney resorts (AKL, All-Star Music, CBR, Contemporary, POFQ, and POP), along with the Swan and Dolphin and several Downtown Disney resort area hotels (Best Western, Hilton, Royal Plaza, Wyndham). Many times, you can get the Swan and Dolphin on special through Allegiant. Last time I checked, Allegiant does not offer Disney tickets, but does offer tickets for Universal and SeaWorld.
Alamo is Allegiant's car rental partner.
As for the whole MCO vs. SFB debate: I liked Sanford, even with the extra distance involved. Everything was close and convenient, security was quick (much quicker than MCO), and the aforementioned Alamo counter was a very short walk from the terminal. The gas stations nearest SFB don't rip you off like the ones at MCO, and the 7-Eleven near the airport has good sandwiches if you're in a hurry.
Some other things about Allegiant:
- Fares shown on their website are after a $4 (flight only) or $6 (package) discount if using a debit card as opposed to a credit card
- Speaking of credit cards, Allegiant is the only major airline that does not accept American Express
- Minimum check-in time at all Allegiant airports is 45 minutes prior to departure as opposed to 30 minutes for other airlines
That said, for a forthcoming trip to WDW next month, I will be using Delta instead of Allegiant, as Delta had a lower vacation package price out of the same airport, and because a family member wanted to use Magical Express and didn't want to rent a car. However, I would still use Allegiant again.