I have flown SPIRIT out of DTW many times. They are a no frills airline, so don't expect snacks, pillows or blankets. I have been on both their newer planes that fly 3/3 seating and the MD 80s that seat 2/3. Some of the MD 80s won't take a roller bag as a carry on. (Slope of the overhead) This happend a few times on flights to Denver (when they were flying there) to me. I had to gate check it.
Thye get me where I need to go mostly on time.
The only drawback to flying SPIRIT is having to leave out of the nasty SMITH terminal at DTW. SWA flies out of there too, but uses the concourse that is somewhat cleaner. Last time I flew SPIRIT, there weren't ANY working monitors in the concourse at all! Baggage claim at DTW can take forever, so be prepared. They use the B terminal at MCO (some of the old USAir gates)
That said, I am going to use them in 3 weeks because I caught a sweet deal on airfare with them.
pinnie