fares are not all nine dollars--few and far between but if the dates work for you then it is really true we have used from chicago to ft lauderdale last winter and it was a great getaway the airline is not very fancy and the seats are very close together but it was a plane and got us there and back safely for a very affordable rate you also have to pay for your luggage (one carry on free) so we packed very lightly and pre-purchased our luggage when we booked our flight--it is cheaper that way it could actually cost more for your luggage than you to fly
Also be aware that Spirit charges for EVERYTHING checked luggage (BTW the Spirit checked luggage weight limit is 40 lbs NOT 50) and carry ons. Figure these prices into the fare to make sure they really are the cheapest option.