I originally reserved the ski equipment through the Breitfuss Intersport shop in Mittersill, to collect at 16:00 on Saturday 26 December. Unfortunately, they were not open when we went there (really? on a Saturday? in a ski-resort?), so we went to the other Breitfuss shop at the PanoramaBahn ski lift. This was a much better outcome. Not only were they open all day (including weekends!), but the staff were very helpful and the equipment was good. We also rented ski lockers to store our skis overnight, so that we didn't have to carry them on the train back to Mittersill.
I would definitely recommend the Breitfuss Intersport ski rental at the PanoramaBahn to everyone. I'm not so sure about the Breitfuss shop in the Mittersill centre, as they were not open when expected.
Richard, London, January 2016