The Megabus service is an efficient and very cheap way to get from the Rosa Parks Transit Centre in Detroit to Union Station, Chicago if you book well ahead. It cost $12US each for us to take the bus because I booked a couple of months ahead.
The bus has the suspension of a brick building, but you can overlook that if you are inclined to focus on the positives.
Clean bus, very polite and helpful staff, and some lovely scenery along the way.
Don’t rely on using that little bathroom at the back of the bus, though, unless you are short and thin with thighs strong enough to keep you on the potty while the bus is bouncing and shuddering.