Be afraid - THEY ARE BREAKING THE LAW and won't adhere to FAA regulations. I will be suing and reporting them and particular employees that are ignorant and refuse to listen. They charged me for my guitar and I know my rights. I tour all over and this was my first time flying with Spirit. NEVER AGAIN!
Here's the law as of 02/14/2012:
FAA Air Transportation Modernization and Safety Improvement
(Sec. 713) Requires an air carrier to permit an air passenger to carry a violin, guitar, or other musical instrument on a passenger aircraft without charge if it can be stowed safely in a suitable baggage compartment in the aircraft or under a passenger seat. Sets forth requirements for the carriage of musical instruments as checked baggage or as occupants of a purchased seat.