Make sure to account for all of the fees with which Spirit will hit you. These include: boarding pass fees, fees for every carry-on (which, iirc, are around $50 per piece), and fees for priority boarding or seats (standard to most carriers now, of course). If you account for one carry-on and a personal item, and perhaps a printed boarding pass — because you didn’t download their digital app — your fare might be about the same as other carriers.
You’ll also have to endure them dropping leaflets on you mid flight as they hock their Spirit Air credit card. The card probably gets you amazing rewards like one free drink per 1.5 million miles flown with them.