Spirit is a relatively low cost, no frills airline, and as others have said, it's fine for a short flight, and as long as you are aware of all the fees, there should be no nasty surprises. When everything goes normally, they are fine. However, if there are any type of irregular ops (weather delays, flight cancellations, flight crews timing out) that's when you can run into issues. Customer service is less than stellar, it can be difficult to reach CS, there are no interline agreements, and there can sometimes be only a flight or two per day, so you could be waiting a LONG time to get re-booked.
We flew Spirit once years ago for a long weekend trip, and there were significant delays on both the inbound and outbound legs of our trip, significantly cutting into our vacation time. I have not flown with them since. Right now the only way I would fly with them, is if we were flying to FL to stay with my parents, so we would not be out any additional money for non refundable hotels, etc.