I haven't flown them, but a client of mine did. Well, she tried to!
She had booked a trip to the Bahamas, and was going to fly Spirit from Chicago to Miami, then switch to American to continue to the islands. But Sprit was delayed 4 hours, and they had no way of making their American flight. Since Spirit has no ticket agreements with any of the other carriers, they would have had to buy a new ticket (at their own expense, Spirit said they wouldn't pay for it even though it was a mechanical delay) to make it to Miami for their flight. But a last minute ticket was over $800 PER PERSON, which my client couldn't afford. So they canceled the weekend trip, which was a surprise for my client's husbands birthday.
To add insult to injury, it took over 6 months for Spirit to refund the un-used ticket. I called once a week for 6 months. They kept saying it was on the way, it had been taken care of. But the credit never showed on the clients credit card. I finally got a nice, competant person on the phone who got it taken care of.
From what I've heard they are a good airline to fly. But if there is a cancelation or delay, your stuck until Spirit can get you out, they won't put you on another carrier.