I would consider Spirit Airlines more like a bus than the airlines of yore. Basically, it is transportation from point A to point B, with frills available at a fee.
That said if it meant riding Spirit or no Disney, I would take Spirit. In your case, the difference is the cost of time. That said, I would rather fly Spirit than drive, mainly due to the time I would get back in the faster travel.
You can see the total cost of the price, with baggage, by pricing the flight to the near end on their website.
As for terrible, they are not terrible. They provide a basic service for a lower fee than their competitors. If this is what you are looking for in travel to Orlando (or anywhere), Spirit is a safe way to travel. However for me, travel is part of the fun of the trip, and I prefer to get a few amenities when I fly.
Spirit has the smallest seat pitch (from back of one seat to the front of seat behind it), which is fine for a relatively short flight, but coming from Oregon, I like to have a little more. Even though I bring my own snacks and beverages onboard, I like the option to have free soft drinks and water. I also enjoy the network and relationships larger airlines have to help get me to the destination during irregular operations.
As such, please take the comments made for/against Spirit, especially by me, in the context made. Spirit is a fine airline for transportation, depending on a person's needs and abilities. Again, I would fly Spirit instead of driving if it meant saving a day for travel.
Hope this helps.