I only have one experience with them, and it was less than stellar all the way around. BOS to MYR for a long weekend.
DS and I were booked on the 10am BOS to MYR, non stop. The flight crew did not show up, and our flight was cancelled, 1 hour prior to departure. We wound up on a US Airways flight, connecting through Charlotte, and did not arrive until after 7pm, so we missed our entire 1st day.
Flying home, there was fog between MYR and BOS, and our flight was delayed several times. That's not a problem, the problem was there was NO ONE from Spirit anywhere to be found to answer questions, or update us.
I found their customer service to be non existant, but the flight itself was fine, once we got in the air.
If given a choice, I choose NOT to fly Spirit.