I flew on Spirit for one trip, and that was 1 trip too many. There were extremely long lines on both ends for check in, the plane on the outbound flight was seriously in need of a good interior refurb, and on returning to Detroit, we waited nearly 2 hours for our luggage. (For those who are familiar with Detroit, even Northwest can usually manage to make bags appear within 45 minutes)
Flint airport is a nice small airport. Very clean and attractive. Not a large selection in terms of food and beverage options, but what is available is reasonably priced. I'd fly out of Flint again with no hesitation.
CAP - I don't think the MD-80 is a variant of the 717. MD-80 is a McDonnell Douglas aircraft, the 717 is made by Boeing. Both, however, are pretty reasonable aircraft.