The only downside to service at DTW is that most carriers (other than Delta) have to use the North terminal; while it's new (and closer to the rental car facilities), it is nowhere near as luxe as the McNamara. Nor is there any decent transportation aside from shuttle busses between the two. If you're using the Westin Hotel, it's in the McNamara as well.
If Spirit can carve out a niche there, why can't anyone else? Seems that they conflict with Delta's routes. When I lived in Ft Lauderdale and had to commute to Detroit, often (then Northwest's) fares were double or triple those of Spirit.
Anyhow, good luck to JetBlue in Detroit, and I certainly hope they can thrive and expand.