We are flying with Spirit for the first time next month out of DTW. As far as the checked baggage charges, you can pay $5 per checked bag one way online before you get to the airport otherwise it is $10 per bag at the airport checkin counter.
Plus, I just got a couple of "Park, sleep, fly" airport/hotel deals recently through the mail.
La Quinta offers 8 days free parking with $99USD/night + tax. Phone is 734-641-9006. Deal expires 12/31/2007
Four Points Sheraton also offers 8 days free parking with $129USD/night + tax. Call 734-729-9000. Also expires 12/31/2007
I believe both hotels are near the Qwik Park on Merriman (888-844-park). We have used Qwik Park and it was VERY good. Pickup was immediate, the van was following us as we went to park on the lot even at the early hour of 5am. Very short ride to the terminal. They have 24 hour service to all the terminals.