I spent a fair bit of last year doing supply work at a school that was Ofsted rated 'outstanding' in all categories. The school had about six cover supervisors, and I was basically the only supply teacher.
I will start out by saying that I had immense respect for the cover supervisors. They (the ones that stuck it out) did a wonderful job, if a bit battle weary by the summer term. Three of the six had teaching qualifications but unable to find their first job.
As a supply teacher, I would say that I had more confidence to adapt the instructions left by the class teacher. The cover supervisors were not really allowed to do this. I also had had a lot of training in classroom management (in my PGCE year, all of my classes were supervised, right up to the July). Cover supervisors had the advantage of being on staff, being known to the students, and being in the loop with disciplinary procedures.
Schools will generally share the good vs bad classes around their teachers, cover supervisors and supply teachers. However, much of the experience could be poor. A lot depends on how well you are able to dust off poor classes and look forward to the next.
The best CS that I came across were those who had ambitions to move on to the next step. They pretty much only had plans to do the job for one year, and then either teach or go onto teacher training. They basically used their CS year to build up their classroom management portfolio, much as I did as a supply teacher.