The seats are hard, there's no legroom, bags are extra, drinks are extra, paper tickets are extra, reserved seats are extra. Their on time arrival is about the worst in the industry 23% (but that's relative to the best being around 15%, SW is 21%). Overbooking compensation is around 5 per 10,000 pax, which is in the middle.
I've flown them a few times, and each time I have regretted it for one reason or another. The worst part is probably the quality of other passengers due to the mistaken belief that Spirit is the cheapest option, but after all fees that's often not the case. Basically, it's a flying bus that will probably get you there, but you won't be singing any Christmas songs. I wouldn't go as far as saying I would definitely book a connecting flight, but I would be generous with my price and time trade-offs.