I had done a trial stimulator last year. I have had spinal fusion (rods & screws) almost 5 years ago. Not sure is that had anything to do with the failure of the trial; but stimulation never reached below my knees. My worst burning pain was in my feet and ankles.
There has been instances reported from some of the people that had a successful trial; but when the permanent stimulator was implanted, they were unable to achieve the same results in spite of reprogrammings of the stimulator. This had been due to scare tissue forming around the leads which eventually moved the leads from their previous position.
There have been success stories as well. It is a fact though that scar tissue will form around the leads and hopefully they do not move from the original intended area.