We've flown Spirit several times to Florida from Detroit. No problems of any kind. I always compare their fares with Air Tran flying out of Flint, Michigan. It is a regional airport that is much closer to us with closer, cheaper parking and a small terminal that's easy to get around. Air Tran is always my first choice, but I will go with Spirit if the fare is low enough to cover the extra parking cost extra driving.
An example. We are flying into Tampa for our next cruise (visiting In-laws first) for $80 one-way, all taxes and charges included. We are using Air Tran rewards flights out of Fort Lauderdale for the return flight. They also have a great rewards program and the Air Tran Visa you can earn rewards on as well. That is how we got the freebie flights. Otherwise the tickets would have been around $200 each which is pretty typical out of the Detroit metro area. I try really hard to get it less than that.
Here's a trick you can try with Spirit and Air Tran BTW. Their prices go up as the plane fills. So if you are booking 4 seats, check pricing for flights for 1, 2, 3, and 4 passengers. Sometimes it pays to split the booking up. If the price goes up with passenger 4, but you are booking them all together, they will charge you the higher price. I found this out b/c I always check the price for one ticket first to find times, cost, etc. Then I put the 4 in and my lower price fare disappeared. Have saved a few dollars booking this way, but haven't had to do that in a while.
More than you wanted to know, I'm sure.
Have fun!
Juli