I can report that the CTA lot at Rosemont is a great option that suited my needs just fine. I needed a place to leave my car for 12 hours or so overnight each night, and the lot only cost $5 for 0-14 hours (14-18 hours costs $8, 18-24 hours costs $12). It was busy, safe, my car was fine, and I felt completely comfortable walking there from the hotel at 6 am by myself. There are lots of people out jogging and walking to work at that hour, as there is an active bus depot and train station at the lot. On a map, it looks very pedestrian unfriendly as you have to walk under an overpass inbetween the lot and the hotels like the Hyatt, Hilton, Intercontinental, etc. that are south of the lot, but the sidewalks are wide and there are tons of people out going about their regular business. It took me about 10 minutes brisk walk from my room to the lot. Don't know if it's doable in the winter when there's snow, but this time of year it was fine. It saved me $17 a day on parking fees! All you do is find a spot, check the number of the space that is posted overhead, and go to the pay station over by the train station entrance. The machines take cash or credit cards (they probably take debit too, but I didn't specifically check that), you enter the number of your parking space, pay for as many hours as you want, and you're done.
Edited: 10:44 pm, August 25, 2012