Spirit is fine. But if you have any customer service problems it's somewhat of a nightmare. There is no place to call, you MUST e-mail, and they get back to you when they feel like it. I've ultimately gotten satisfaction, but it was very stressful to get them to follow-through with what they promised me when I took a voluntary bump on a flight. They promised me 2 r/t tickets, but when I tried to use them they first told me it was only 2 $50 credits, then when they admitted it was 2 free tickets they tried to tell me that I needed to use them both on different flights, that my wife couldn't use one of them, and so on. It took a couple of weeks and a lot of argument and stress, but we finally got what they promised us.
I will NEVER take a voluntary bump with them anymore, but I'll certainly fly Spirit again. In fact, I'm using those free tickets in a few weeks.