I used to fly them once a month from DFW to MSP and back. If you are a novice passenger to airlines, they may not be for you. Know the rules, follow the rules, don't expect more than a non-recline seat(Spirit says they are "pre-reclined" tongue in cheek), access to a bathroom, and some small space in your footwell for a bag.
EVERYTHING else comes at a price. I mean - everything. I could pack my carry-on for a long wknd and fit it under the seat, so no charge. I paid for a beer a couple times, and it was same price as other carriers. Planes are in nice enough shape, people were at least as friendly as AA or SWA, and certainly better than UAL staff. They are good at ticketing. They are not good at irregular operations. If something goes wrong along the way, expect to have 'issues'. The plan is to take non-stops unless you can live with some drama. Miss your connection, and you will most likely be paying another fare charge(but not always) to get you to your destination.
As I've never checked a bag, I can't comment on that. Just don't expect any extras besides the seat, footwell, and bathroom access. It is the Greyhound of the skies.