I have had good to OK experiences with PreFlight O'Hare. The parking deck seems safe, stocked with bottled water in the lobby, good elevators, and the shuttle service TO the airport is generally quite good. LEAVING the airport returning home from the Intl terminal I have had some problems with full shuttles.
I have also done sleep, park and fly at the 4 Points Sheraton, which is directly next door to PreFlight, and also the Wyndham O'Hare. 4 Points always put me in a secure fenced lot. Wyndham put me in an "extension" lot away from the hotel and simply an open lot behind an office building. You need to sign up for parking ahead of time, and generally 7 days included with your stay. You also have to book your departure shuttle time with the desk (have always done it at check in) so you get a spot on the shuttle. The last time I did this I stayed at Wyndham and ended up just getting an Uber because getting a shuttle back to the hotel was such a hassle (seems to be a problem with everyone I've tried). I used parksleepfly.com to book when I did Wyndham in January and that was a pretty seamless way to book a hotel and parking.
The neighborhood around PreFlight and 4 Points (South of the airport) seems to have taken a turn for the worse. I haven't had any problems there, but there have been some unsavory looking people hanging around that area (particularly the gas station right there) that made me question whether things are going the wrong way in that neighborhood.
My experiences with the "cheap" airport lots is while cheaper it takes a long time to get to and from the airport on the tram, the lots are gigantic.
O'Hare is easily the worst airport in the US I have ever been to. Dirty, disconnected, roads and trams shut down for repairs/construction, and generally badly designed.