We have flown Allegiant twice - once to Orlando (Sanford) and once to Las Vegas (which then we drove on to Anaheim).
Yes, they are cheaper from the get-go (our Orlando flight was under $100 round trip!) but then you do have to factor in car costs to get you to WDW since there's no Magical Express from Sanford. Plus all the ways they nickle and dime you to death for luggage, seat selection, etc. Also, with their limited schedule if something goes wrong, you're up a creek. And their customer service is less than desirable if you start looking at reviews.
Both times we lucked out. It was still cheaper for us to fly from our small regional airport and rent a car once we got to Orlando the first time. Otherwise we have to drive 2.5 hours to the "big" airport, pay to park the car for a week, etc. They no longer offer service to Orlando so it's not an option anymore for us to use Allegiant.
And when we went to Vegas/Anaheim last January, it was our personal challenge to see how cheap we could do the trip - I had won tickets to
Disneyland, a free night's hotel stay, we used our DVC points for the other couple of nights... flying Allegiant meant not driving the 2.5 hours, my parents dropped us off so no parking fees, and we didn't pay anything extra on the tickets since it was me, my DH, our DS11, and his friend. If we got split up on the plane we all would have been fine. So we all packed our stuff in our one free personal carryon that sat at our feet! Our backpacks were bursting at the seam, but we made it work for our 4 night trip! I was nervous flying in the winter that the flight would be cancelled and then we'd be out of luck (and we actually flew out on a day it was -30F and the govenor cancelled school for the whole state of Minnesota) but we left on time and had a good flight both ways.
Anyhow, there's my Allegiant stories!