After almost vowing not to fly Spirit due to personal experience and even worse experience of my BIL, I did actually fly Spirit once, after reading DDMS post about beating Spirit at their own game, and buying at the airport.
I had a DL ticket for the route (LGA-FLL) booked through an OTA that allowed for cancellations. I walked up to the Spirit ticket counter, made sure that flight was actually leaving (wouldn't buy Spirit in advance), made sure Big Front Seat was available, purchased the ticket and then canceled the DL flight for a full refund.
IMHO that is the only way to fly Spirit (and due to their PUC game, it actually ends up cheaper).