Sanford airport is nice. It's fairly small, which can be good or bad depending upon the timing. If yours is the only plane leaving or arriving around a particular time (which is usually the case), there are very few people in the terminal. That means no lines for security, baggage claim, car rental, etc. However, if there is another flight or two, things back up quickly because the airport is just not designed for that kind of traffic flow. So, you'll want to allow yourself a few extra minutes for check-in/security on your flight out just in case. Also note that the food at Sanford is somewhat limited so plan accordingly. I think they have a little snack shop and that's about it.
We've rented a car each time we've flown into Sanford and found the process easier than at Orlando. The drive to Disney from Sanford is very easy - basically you just want to stay on 417 all the way to the Disney exits. Note that some GPS systems will try to steer you back to I-4 (via 408 I think) - do NOT do this! If you stay on 417 it's another 30 minutes or so of easy driving compared to Orlando International - not a big deal at all.
We've flown Allegiant and had no issues. As someone else said, their prices are good even with extras added in. BTW, I've never paid extra to choose my seat and never had a problem - I got the feeling only a few people fall for this.
Personally I would take a direct flight to Sanford over a connecting flight through Atlanta or Charlotte to Orlando EVERY time. (I spent a night stranded in Atlanta with my family once - thanks Delta!) When comparing price and my desire for a direct flight, basically I choose between Allegiant direct to Sanford or Southwest direct to Orlando. Allegiant is usually cheaper but is limited on flight times. When I go this year I'm flying Southwest because Allegiant didn't have a morning flight and I didn't want to lose an entire day of my vacation.
Good luck!
