We've flown from Detroit to MCO via Spirit roundtrip 4 times. Like others have stated, it is definitely a no frills airline...they charge for carryon, checked bags, as well as seat assignments, water & snacks on the flight. Be sure to add up all the hidden costs to make sure you are really getting a cheap flight.
That being said, we have never had a bad experience with them. They leave on time, they arrive on time, they don't tend to move flight times around like other airlines I read about on here, and they have low cost nonstop flights with times that are desirable to us.
We've never paid more than $700 for nonstop 4 RT tickets. That price includes the cost of 2 checked bags and no carryons. We leave Detroit on the earliest flight and arrive in MCO by 10am and have a full day of vacation our first day. They also have late flights leaving between 8 - 10 pm so you can have a full last day as well. I have found if you do not pay for seat assignments, but check in right at the 24 hour mark, you will be seated with your family members.
I've read alot about their poor customer service, but never actually witnessed it. What we have gotten is really affordable transportation.