I just signed up for AutoCheck (run by Experian) and it 'works' in the sense that it lists things that were reported. I've been talking with a guy about a car, and he says it looks like there was some minor dent/scratch on the car that was repaired, but it doesn't show up in the report. Assuming the speedo wasn't changed or broke, or whatever, it also gives you a good idea about how the car was driven. In my case, this car had 120,000 miles in the first 4 years, and then 8 over the next 5, then 8 over the next 7, then 9 over the past 2. I think the only thing that it will really uncover is that the car hasn't been salvaged. You'll also find out when and where the car was registered over it's life. Certainly I wouldn't buy a car solely or even mostly based on the report. It might just be a place to find something to question. That's what I did with the mileage that I mentioned. Only the 120,000 figured was in the report, but the current owner gave me the history about the rest of the mileage.
I went with AutoCheck because it was cheaper $25/60 days and on the 'free prelim' report for 1 car it said there was 11 items to report, whereas CarFax said it had only 4.