Other than their a-la-cart price structure (which people should understand before buying), the big issue with Spirit is that they don't have reciprocity agreements with other airlines. That means if a flight gets bumped, or you get bumped off a flight, you're stuck until the next available Spririt flight. They are not obligated to book you on another airline. So, if flying Spirit, you want to make sure you're flying out of their hub airports, otherwise it can get ugly.