This article is good info for those who are Spirit-curious. Spirit could be a great option for those adventurous souls who are somewhat flexible and just want cheap transport from A to B. If your expectations are aligned, you should be fine.
Spirit essentially views their pricing model as one where everything is extra (“Bare Fare”), so that those fliers who, for example, do not check a bag are not subsidizing the cost to provide this service to those who do.
Spirit is also pressuring the pricing of the US legacy major airlines.
Also, check out this entertaining video explaining their pricing model: