I did in fact have the opportunity to depend on one of these systems in my not so long ago days in retail. The system was originally installed with 4 rails, one in each leg of the "racetrack". The ability to literally follow someone throughout the store was indeed tremendous. However it did have limitations, You could not depend on this system for monitoring, it was strictly for tracking subjects. We still depended on fixed cams to monitor high risk areas. Store layout also impaired the effectiveness of the system. Ailses werent lined up perfectly in a row up and down. The rails couldnt see everything so we added several traditional PTZs to supplement. As already mentioned, over time noise became a large concern. It didnt help that the tracks werent that high, only about 10-12 feet up. We had them serviced every other year, but after time, even new wheels didnt quiet them down anymore. Customers rarely took note of the funny sound overhead but the employees ALWAYS knew if we were watching with the rail. Travel time was also now something we had to consider. Travel time from one end of the store to the other was about 30 something seconds, which was long enough to loose someone. I set presets at each end and the middle so I could direct the opposing rail to be closer in case the subject being tracked moved across.
As a user, I loved it. But like many things CCTV, it wasnt an all in one solution.