I recently did a job shooting a Sony EX-3 and used a teleprompter that was supposedly appropriate for HD. The teleprompter performed well, but I notice slight degradation of the image quality. Certainly usable, but I'm wondering if there are particular units with higher quality glass/beam splitters?
Was it a true "first surface" mirror, is the first question. Those are much more expensive.
Was the glass clean?
Was the camera lens as close as possible to the glass without touching it?
And was the light-blocking cloth well-sealed to prevent any reflections or glares inside the unit?
Was the text cranked up too bright so that it was noticeable crossing the lens?
Was the black level of the LCD or CRT source set as dark as possible?
You should expect to lose around a full F-stop shooting thru a prompter, and you need to adjust iris for that.