I am a happy customer with Spirit. But, I do want to note that when booking using their travel miles, be careful. I have noticed that when I select flights using few points and don't purchase immediately, the miles requirement goes up. My theory: you sign in to your account, start browsing your flights, perhaps leave the site to come back later, sign in again, and voila, your flights have gone up. The computer systems are seeing your thought process, then upping it. So, begin with a set plan in mind and book it the first time you log in or suffer the possibility of the flight miles going up. I have done this repeatedly and get the same issue.
I now sign in only when I am actually really ready to book so the system can't track me. Just a tip!