I flew Spirit from DTW last May with the family and the flight itself left at like 7am so we had no problem with delays.
I will say that they had a confusing ticket area, with 4 different flights appearing to be in the same line (no signs telling you where to go for the different flights) so I stood in the wrong line for about 15 minutes at 6am! (and I am NOT a morning person) Then a little lady came around to double check people were in the right line and redirected me. It was chaos.
I am actually debating paying more for Northwest because of the better times they have. I am NOT waking up at 5am with my children EVER again!
Right now, NWA is running awesome specials, my neighbors got roundtrip DTW to MCO in August for $136.00 That is a GRREAT price, it even beats Spirit! Of course, when I need it in October they aren't offering it!
Good luck, I am playing the airfare game right now too!