I was recommended to SpotHero.com by a friend (who had used it). I had a hotel in the Magnificent Mile zone that wanted to charge $52 per night for parking. I went to SpotHero and put in the hotel address and was notified of several parking locations for the night I would be there of $10 and up. The cheaper one was a small surface lot. I chose a garage on Grand Avenue (one block from my hotel) for $18.50. They charged my credit card in advance. The SpotHero printout gave excellent instructions to the parking complete with photo of the place and detailed instructions of how to find it and what to do when I get there. You can also show them a picture of the confirmation email on your cell phone if you want - it has a QR code that the garage can scan (I think). When I went to check out first I brought (walked) the SpotHero confirmation paper to the office but they said that I should drive there because once they validate my parking ticket, it's only valid for 5 minutes. So we drove to the exit, handed the paper in, she validated our ticket, I shoved it into the machine, the arm lifted and we were on our way. We saved over $30 using SpotHero.
So in case anyone else is wondering (since I see SpotHero is starting to advertise now) I can say that in this (my first) case, I was happy. My friend was also happy also. He called them to ask something and found the people very helpful. You can call or chat electronically with them if you want. I would use them again and would recommend SpotHero to others.