several threads already on Spirit Airlines, so I am locking this thread.
Please use search: a simple Spirit Airlines
brings up several reviews as well as questions and answers (along with usual unrelated threads).
If you have questions feel free to ask them in existing threads.
I'll give you the quick an dirty on NK: cheap, sometimes very cheap, but you pay extra for luggage and even carryon. Service can be horrible at times.
Also interesting reading in Wikipedia