We have flown Allegiant to the Sanford airport and Delta & American to MCO. Here are some things to keep in mind....
Allegiant flies in the SANFORD airport ... not MCO ... so you do not have access to the free ME bus system. You will need to either rent a car or pay for a taxi or towncar. In our experience, it was cheaper to rent a car than pay for the latter. For that trip, we still saved money with the rental car figured in ... and we ended up with a very sick kid, so it was great to have a car.
Allegiant has tons of hidden fees. Be sure you figure them in. When it has a super deal (say $39.99), in reality that is at the very least $80 - more if you don't opt out of all the add-ons. One way to avoid at least ONE of the hidden fees is to actually go to your airport and book it. They charge a fee for you to book on your own computer or over the phone. Baggage is very expensive too, as well as seating fees, etc. Also, say you are only taking 1 bag down, but may have 2 bags coming home, pay for that when you book. If you add on another bag at the airport just before you fly home, you'll pay a lot more.
It can still be cheaper than other airlines, but you need to do the research. We loved the direct flight too. As another poster mentioned, all snacks & drinks are extra on the flight. Think movie theater prices. We take along snacks and buy drinks at the airport before we board.
On another trip, we did better by driving 3 hours to another airport and flying Southwest, saving $400 over the best Allegiant deals. We also enjoyed the ME bus service and did not need a car that trip. Saved the hassles of dealing with Orlando traffic.
I watch the Allegiant deals to LV as we have family there. From what I've seen, they have their "regular" prices 5-6 months out, then rise a bit, then drop, then raise substantially the last week or two before the date. You have to watch often ... and then jump on it ... because they are just as apt to change within even minutes. Yes. Really. Been there.
Have fun planning your vacation!