I fly Spirit from Detroit frequently. The last few times, there has been some sort of flight delay. Don't sweat it if the plane is a bit late.
Spirit has a new fleet of Airbus planes. I think there is more room at each seat than in Boeing planes. Just don't sit in the aisle ahead of the exit rows since the seats don't recline.
Since February 2007, they started to charge $10 to check a second piece of luggage. I think they will start charging $10 for each bag checked starting in June. Keep this in mind when booking flights.
Parking at the Blue Deck by the Spirit terminal is a good value at only $8/day.
Since Spirit started charging for checked bags, I switched back to Northwest. They frequently match Spirits low prices. We typically pay around $110-170 after taxes for our flights on either company.
Make sure to sign up for Spirit's emails so you can be notifed of the $9 flight sales. We are flying to Atlantic City for only $44 after taxes!!