I have no direct experience of the OU, but I did have a mathematics teacher (head or deputy head I think of the department at a large school, and taught the more advanced courses) who used it.

Not taught courses, took its courses himself, in mathematics.

It went up in my estimations then for sure, because he was already a great teacher, intelligent and knowledgeable in the subject (certainly more than enough for up to 'Further Additional' at A level) and studying for his own interest (or I suppose a career change or further study for all I know).