Downtown is very safe. Unless you are afraid of clean towels then you have nothing to fear. All of the real crime in Detroit happens in the neighborhoods. Don t get me wrong, it is a big city and anything can happen, but less likely in downtown. I lived downtown and it is not like some stupid tv show. At night everyone is gone. You actually have a few places that stay open all night that are just cool dives to stop in if you want a story to take home. Nothing violent, but amusing. You will find that most of the downtown is filled with working people during the day, party kids at night, and mostly quiet empty streets at night. If you want somewhere dangerous to stay I can recommend a few places, but Marriott is not one of them.
http://www.marriott.com/hotels/travel/dt��It is inside the darn Renaissance Center ... the only place safer would be Fort Knox.
I have stayed there numerous times, and have never felt unsafe despite what the perception of the Downtown area may be.
It is absolutely safe. I ve stayed there several times for conventions.